0.15.0: stage context menu + drag-to-resize stage

- Right-clicking the stage opens a small menu with Document settings
  (pops the document drawer) and Resize stage. Closes on outside
  click and ESC.
- Resize stage drops eight accent-coloured drag handles around the
  canvas rect (N/NE/E/SE/S/SW/W/NW). East/south handles just grow or
  shrink the canvas; west/north (and their corners) also shift every
  scene-world coordinate in lock-step so content stays pinned to its
  visible position while the canvas re-aligns around it.
- Enter forces stage-crop on so the user can see where the new edge
  lands; exit restores the previous crop setting. Exit paths: Done
  button in the floating banner, ESC, or the restored crop toggle.
- New store action shiftSceneWorldCoords(dx, dy) covers layer base +
  main/position keyframes + bezier control points + clipInstance x/y
  across every scene page. Clip-library layers, button assets, hit
  shapes and polygon vertex clouds stay put — those live in local
  coord systems, not scene-world space.
- Tests: store coverage (shift across pages, positionKeyframes,
  ctrlX/Y, clipInstance, clip-library untouched, (0,0) no-op) plus UI
  coverage (context-menu items, Resize stage enters mode + renders 8
  handles, ESC and Done both exit, crop forced on and restored).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/stores/scene.ts

@@ -776,6 +776,46 @@ export const useSceneStore = defineStore('scene', {
       this.project.canvas.width = Math.max(1, Math.round(width))
       this.project.canvas.height = Math.max(1, Math.round(height))
     },
+
+    /** Shift every scene-world-space coordinate across the whole project
+     *  by (dx, dy). Used by the stage resize drag: when grabbing the
+     *  left/top edge the canvas origin effectively slides, and every
+     *  element needs to slide the same amount to stay visually pinned.
+     *
+     *  What shifts:
+     *   - layer.base.{x,y}
+     *   - each keyframe.props.{x,y} on the main track
+     *   - each keyframe.props.{x,y} on separated position track
+     *   - keyframe.ctrlX / ctrlY (bezier handles live in world space)
+     *   - clipInstance.{x,y} (static per-instance offset)
+     *
+     *  What stays put: clip-library layers (local to the clip instance
+     *  origin), button-asset internals (their own canvas), polygon point
+     *  clouds on keyframe.props.points (local to the layer origin). */
+    shiftSceneWorldCoords(dx: number, dy: number) {
+      if (dx === 0 && dy === 0) return
+      const shiftProps = (props: Partial<AnimatableProps> | undefined) => {
+        if (!props) return
+        if (typeof props.x === 'number') props.x += dx
+        if (typeof props.y === 'number') props.y += dy
+      }
+      const shiftKeyframe = (kf: Keyframe) => {
+        shiftProps(kf.props)
+        if (typeof kf.ctrlX === 'number') kf.ctrlX += dx
+        if (typeof kf.ctrlY === 'number') kf.ctrlY += dy
+      }
+      for (const page of this.project.scenes) {
+        for (const layer of page.layers) {
+          shiftProps(layer.base)
+          for (const kf of layer.keyframes ?? []) shiftKeyframe(kf)
+          for (const kf of layer.positionKeyframes ?? []) shiftKeyframe(kf)
+        }
+        for (const ci of page.clipInstances ?? []) {
+          if (typeof ci.x === 'number') ci.x += dx
+          if (typeof ci.y === 'number') ci.y += dy
+        }
+      }
+    },
     setCanvasBackground(color: string) {
       this.project.canvas.background = color
     },