feat: text tool UX, image crop, text format toolbar, double-click focus

- Text tool: click-to-place immediately opens inline editor, auto-switches
  back to select tool. Empty text cleanup on save.
- Crop: select image + press C (or right-click > Crop) to enter crop mode.
  8 drag handles with rule-of-thirds grid, dimmed outside area.
  Enter confirms, Escape cancels. Non-destructive (stored as normalized rect).
- Text format toolbar: appears when text items selected, with font size +/-,
  font family dropdown, and color picker with presets.
- Double-click image: zoom-to-fit (PureRef-style focus)
- Entrance animation: replaced bounce with simple fade-in (no delay before
  items become interactive)
- TextEditor: allow empty text (caller handles cleanup)

frontend/src/canvas/SelectionManager.ts

@@ -64,6 +64,7 @@ export class SelectionManager {
   private static readonly DOUBLE_CLICK_MS = 400;
 
   private _onDoubleClickText: ((item: SceneItem) => void) | null = null;
+  private _onDoubleClickImage: ((item: SceneItem) => void) | null = null;
 
   private _enabled = true;
 
@@ -142,6 +143,11 @@ export class SelectionManager {
     this._onDoubleClickText = fn;
   }
 
+  /** Called on double-click of an image item (zoom-to-fit). */
+  set onDoubleClickImage(fn: (item: SceneItem) => void) {
+    this._onDoubleClickImage = fn;
+  }
+
   /** Select only this item, deselecting everything else. */
   selectOnly(id: string): void {
     this.selectedIds.clear();
@@ -357,6 +363,8 @@ export class SelectionManager {
           item.displayObject.togglePlayPause();
         } else if (item.type === 'text') {
           this._onDoubleClickText?.(item);
+        } else if (item.type === 'image' || item.type === 'drawing') {
+          this._onDoubleClickImage?.(item);
         }
         this._lastClickTime = 0;
         this._lastClickItemId = null;