feat: zoom-bucket text sharpness + remove fontSize toolbar controls

Add TextSharpnessManager for crisp text rendering at all zoom levels:
- Discrete zoom buckets (0.5–3x) avoid texture churn on small zoom changes
- Initial bucket applied to all items on setup and newly created items
  via SceneManager.onItemCreated callback (fixes blurry-on-load)
- Visibility check uses getItemWorldBounds for correct grouped item coords
- TextSprite/StickySprite gain setZoomBucket() for resolution control

Remove fontSize +/- controls from contextual toolbar — font size is now
controlled purely through direct manipulation (resize → bake).

frontend/src/canvas/sprites/StickySprite.ts

@@ -49,6 +49,7 @@ export class StickySprite extends Container {
 
   // Background redraw cache: skip Graphics.clear()+redraw when shape is unchanged
   private _bgCacheKey = '';
+  private _zoomBucket = 1;
 
   /** After layout, the computed card height. Parent should sync to data.h. */
   get computedHeight(): number {
@@ -60,6 +61,16 @@ export class StickySprite extends Container {
     this._text.visible = visible;
   }
 
+  /**
+   * Update text rasterization resolution for zoom-bucket crisp rendering.
+   * Only re-rasterizes when the bucket actually changes.
+   */
+  setZoomBucket(bucket: number): void {
+    if (bucket === this._zoomBucket) return;
+    this._zoomBucket = bucket;
+    this._text.resolution = bucket;
+  }
+
   constructor(data: StickyObject) {
     super();