feat(measurements): rect click-through, color-as-selection, fullscreen, label clamping

- Rectangles now follow the same body-click rule as circles: corner dots
  drag, body (rim + interior) selects when no tool is active and falls
  through to placement when one is. Lets the user draw measurements
  inside an existing rectangle.
- Selected shapes keep their own colour for the geometry stroke instead
  of swapping to white. Unselected shapes are dimmed to 0.45 alpha so
  the selected one still stands out, and the colour stays consistent
  with the label pill — easier to pair shape ↔ label.
- Label clearance: anchors that coincide with a handle (ellipse, circle,
  angle) now use a larger Y offset so the pill clears the handle disk.
  Inter-label gap bumped from 4 to 8 px.
- Export labels are clamped to the destination canvas after collision
  resolution so full-resolution exports never clip a pill off the edge.
- Fullscreen toggle in the toolbar (icon + text). Esc exits. Renders the
  viewer as a fixed-position overlay; the existing ResizeObserver picks
  up the new container and re-fits the image automatically.

src/components/CorrectedImageViewer.vue

@@ -51,6 +51,13 @@ const gridSpacingMm = ref(10)
 // the nearest 45° (0/45/90/135…) relative to the fixed endpoint or angle
 // vertex. Length is preserved; only direction is constrained.
 const snapToAngle = ref(false)
+// Full-screen mode: render the viewer as a fixed-position overlay covering
+// the viewport. The ResizeObserver picks up the new container size and
+// re-fits the image; nothing else needs to change.
+const isFullscreen = ref(false)
+function toggleFullscreen() {
+    isFullscreen.value = !isFullscreen.value
+}
 
 // Measurement types live in `@/types/measurements` so the cache module and
 // other consumers can share them. Geometry is in image space so it stays
@@ -558,7 +565,16 @@ function labelRect(
     const h = 20 * rt.strokeMul
     const w = tw + pad * 2
     // Offset the label above the anchor so it doesn't sit on top of a handle.
-    const offsetY = (m.type === "angle" ? 22 : 14) * rt.strokeMul
+    // Anchor types whose anchor coincides with a handle disk need extra
+    // clearance so the label doesn't sit on top of the handle. Selected
+    // primary handles are 8 px radius + 2 px stroke, so we need at least
+    // h/2 + 10 + breathing-room. Line midpoint and rect centroid don't
+    // host a handle — they only need a small visual gap from the anchor.
+    const anchorOnHandle =
+        m.type === "ellipse" ||
+        m.type === "circle" ||
+        m.type === "angle"
+    const offsetY = (anchorOnHandle ? 26 : 16) * rt.strokeMul
     const x = anchor.x - w / 2
     const y = anchor.y - h / 2 - offsetY
     return { x, y, w, h, textX: anchor.x, textY: anchor.y - offsetY, fontPx }
@@ -572,8 +588,14 @@ function drawMeasurement(
 ) {
     const baseColor = getDatumColor(m.colorIndex)
     const decorate = rt.drawSelectionDecorations
-    const strokeColor = decorate && isSelected ? "#ffffff" : baseColor
-    const lineAlpha = decorate ? (isSelected ? 1.0 : 0.8) : 1.0
+    // The selected shape always renders in its own colour rather than a
+    // white "highlight" stroke — the colour is what ties the geometry to
+    // its label pill, so swapping it out on selection actively hurts
+    // identification. We dim unselected shapes more aggressively (0.45 vs
+    // 0.8) and keep the thicker, solid stroke for the selected one so it
+    // still stands out without recolouring.
+    const strokeColor = baseColor
+    const lineAlpha = decorate ? (isSelected ? 1.0 : 0.45) : 1.0
     const lineWidth = (decorate && isSelected ? 3 : 2) * rt.strokeMul
 
     ctx.save()
@@ -893,7 +915,7 @@ function resolveLabelPositions(
     list: Measurement[],
     rt: RenderCtx,
 ): Map<string, LabelPos> {
-    const gap = 4 * rt.strokeMul
+    const gap = 8 * rt.strokeMul
     const items = list.map((m) => {
         const anchor = imgToCtx(labelAnchor(m), rt)
         const rect = labelRect(ctx, m, rt)
@@ -923,19 +945,36 @@ function resolveLabelPositions(
         placed.push(it)
     }
 
+    // Clamp every label to the destination canvas so exports never clip
+    // labels off the edge. Done after collision resolution as a separate
+    // pass — clamping can re-introduce overlaps, which we accept since
+    // staying inside the bitmap is more important than zero-overlap on
+    // crowded edges. The live overlay benefits too: labels near the
+    // viewport edge stay visible instead of running off the canvas.
+    const cw = ctx.canvas.width
+    const ch = ctx.canvas.height
     const out = new Map<string, LabelPos>()
     const shiftThreshold = 4 * rt.strokeMul
     for (const it of placed) {
+        let { x, y, w, h, textX, textY, fontPx } = it.rect
+        const dx =
+            x < 0 ? -x : x + w > cw ? cw - (x + w) : 0
+        const dy =
+            y < 0 ? -y : y + h > ch ? ch - (y + h) : 0
+        x += dx
+        y += dy
+        textX += dx
+        textY += dy
         out.set(it.id, {
-            x: it.rect.x,
-            y: it.rect.y,
-            w: it.rect.w,
-            h: it.rect.h,
-            textX: it.rect.textX,
-            textY: it.rect.textY,
-            fontPx: it.rect.fontPx,
+            x,
+            y,
+            w,
+            h,
+            textX,
+            textY,
+            fontPx,
             anchor: it.anchor,
-            shifted: Math.abs(it.rect.y - it.originalY) > shiftThreshold,
+            shifted: Math.abs(y - it.originalY) > shiftThreshold,
         })
     }
     return out
@@ -1708,21 +1747,20 @@ function pointerDown(
     const hit = hitTest(cursor)
     if (hit) {
         const target = measurements.value.find((m) => m.id === hit.measurementId)
-        // Circles are exclusively manipulated via their two handles —
-        // center (which translates the whole circle while keeping its
-        // radius) and edge (resize). Body and label hits don't drag.
-        const isCircleBodyHit =
-            target?.type === "circle" && hit.kind !== "handle"
-        // When a placement tool is active and the hit is on a circle's
-        // body, fall through to placement entirely — don't select, don't
-        // suppress the click. This lets the user draw a new measurement
-        // on top of an existing circle. Handles still claim the press
-        // so the circle can be reshaped from within an active tool too.
-        if (isCircleBodyHit && activeTool.value !== "none") {
+        // Closed shapes (circle, rectangle) are only draggable from their
+        // handles — corner dots for rect, center / edge for circle. Body
+        // hits select but never start a drag, so big shapes don't drift
+        // when the user clicks inside them. When a placement tool is
+        // active, body hits fall through entirely so the user can draw a
+        // new measurement on top of an existing closed shape.
+        const isClosedBodyHit =
+            (target?.type === "circle" || target?.type === "rectangle") &&
+            hit.kind !== "handle"
+        if (isClosedBodyHit && activeTool.value !== "none") {
             return "placement"
         }
         selectedId.value = hit.measurementId
-        if (target && !isCircleBodyHit) {
+        if (target && !isClosedBodyHit) {
             const mode: DragMode = hit.kind === "handle" ? "handle" : "move"
             dragState = {
                 mode,
@@ -1978,6 +2016,10 @@ function onKeyDown(e: KeyboardEvent) {
         if (selectedId.value !== null) {
             selectedId.value = null
             drawOverlay()
+            return
+        }
+        if (isFullscreen.value) {
+            isFullscreen.value = false
         }
         return
     }
@@ -2306,7 +2348,16 @@ watch([viewScale, viewOffsetX, viewOffsetY], () => {
 </script>
 
 <template>
-    <div class="space-y-3">
+    <!-- In fullscreen mode the viewer becomes a fixed-position overlay that
+         covers the viewport. The ResizeObserver inside the canvas picks up
+         the new container size and re-fits automatically. Esc exits. -->
+    <div
+        :class="
+            isFullscreen
+                ? 'fixed inset-0 z-50 space-y-3 overflow-auto bg-background p-4'
+                : 'space-y-3'
+        "
+    >
         <!-- Toolbar -->
         <div class="flex flex-wrap items-center gap-2 text-sm">
             <div class="inline-flex rounded-md border border-border p-0.5">
@@ -2443,6 +2494,52 @@ watch([viewScale, viewOffsetX, viewOffsetY], () => {
                 Clear all
             </button>
 
+            <button
+                class="inline-flex items-center gap-1 rounded-md border border-border px-2.5 py-1.5 text-xs text-muted-foreground hover:text-foreground"
+                :title="
+                    isFullscreen
+                        ? 'Exit fullscreen (Esc)'
+                        : 'Expand to fullscreen'
+                "
+                @click="toggleFullscreen"
+            >
+                <svg
+                    v-if="!isFullscreen"
+                    xmlns="http://www.w3.org/2000/svg"
+                    width="14"
+                    height="14"
+                    viewBox="0 0 24 24"
+                    fill="none"
+                    stroke="currentColor"
+                    stroke-width="2"
+                    stroke-linecap="round"
+                    stroke-linejoin="round"
+                >
+                    <path d="M3 9V3h6" />
+                    <path d="M21 9V3h-6" />
+                    <path d="M3 15v6h6" />
+                    <path d="M21 15v6h-6" />
+                </svg>
+                <svg
+                    v-else
+                    xmlns="http://www.w3.org/2000/svg"
+                    width="14"
+                    height="14"
+                    viewBox="0 0 24 24"
+                    fill="none"
+                    stroke="currentColor"
+                    stroke-width="2"
+                    stroke-linecap="round"
+                    stroke-linejoin="round"
+                >
+                    <path d="M9 3v6H3" />
+                    <path d="M15 3v6h6" />
+                    <path d="M9 21v-6H3" />
+                    <path d="M15 21v-6h6" />
+                </svg>
+                {{ isFullscreen ? "Exit" : "Fullscreen" }}
+            </button>
+
             <div class="mx-1 h-4 w-px bg-border" />
 
             <label