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/TextSprite.ts

@@ -23,6 +23,7 @@ export class TextSprite extends Container {
   private _lastFontSize = 24;
   private _lastFontFamily = 'sans-serif';
   private _lastFill = '#ffffff';
+  private _zoomBucket = 1;
 
   /** Measured width after last text change. */
   get measuredWidth(): number {
@@ -39,6 +40,16 @@ export class TextSprite 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: TextObject) {
     super();