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/i18n/messages.ts

@@ -322,6 +322,15 @@ export const en = {
       addPolygon: 'Add polygon hit area',
       remove: 'Remove selected hit area',
     },
+    menu: {
+      documentSettings: 'Document settings',
+      stageResize: 'Resize stage',
+    },
+    resize: {
+      banner: 'Resizing stage',
+      hint: 'Drag a handle to resize — press Esc or click Done.',
+      done: 'Done',
+    },
   },
 
   settings: {