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/textSharpness.ts

@@ -0,0 +1,129 @@
+/**
+ * textSharpness — zoom-bucket-aware text rendering.
+ *
+ * Tracks the viewport zoom level and groups it into discrete buckets.
+ * When the bucket changes, visible text/sticky sprites are re-rasterized
+ * at the new resolution for crisp rendering. Small zoom movements within
+ * the same bucket are ignored to avoid unnecessary texture churn.
+ *
+ * This does NOT mutate scene data (fontSize stays stable).
+ * It only changes the render resolution of text textures.
+ */
+
+import type { Viewport } from 'pixi-viewport';
+import type { SceneManager, SceneItem } from './SceneManager';
+import { getItemWorldBounds } from './SceneManager';
+import { TextSprite } from './sprites/TextSprite';
+import { StickySprite } from './sprites/StickySprite';
+
+/**
+ * Map continuous zoom to discrete bucket.
+ * Fewer buckets = fewer re-rasterizations = better perf.
+ */
+export function getTextZoomBucket(scale: number): number {
+  if (scale < 0.4) return 0.5;
+  if (scale < 0.75) return 0.75;
+  if (scale < 1.25) return 1;
+  if (scale < 1.75) return 1.5;
+  if (scale < 2.5) return 2;
+  return 3;
+}
+
+export class TextSharpnessManager {
+  private _viewport: Viewport;
+  private _scene: SceneManager;
+  private _currentBucket = 1;
+
+  constructor(viewport: Viewport, scene: SceneManager) {
+    this._viewport = viewport;
+    this._scene = scene;
+    this._currentBucket = getTextZoomBucket(viewport.scale.x);
+    // Apply initial bucket to all existing text/sticky items
+    this._refreshAll();
+  }
+
+  /**
+   * Call on viewport zoom/move. Returns true if bucket changed.
+   */
+  check(): boolean {
+    const bucket = getTextZoomBucket(this._viewport.scale.x);
+    if (bucket === this._currentBucket) return false;
+    this._currentBucket = bucket;
+    this._refreshVisible();
+    return true;
+  }
+
+  /**
+   * Apply current zoom bucket to a single item.
+   * Call this when a new text/sticky sprite is created.
+   */
+  applyToItem(item: SceneItem): void {
+    if (item.displayObject instanceof TextSprite) {
+      item.displayObject.setZoomBucket(this._currentBucket);
+    } else if (item.displayObject instanceof StickySprite) {
+      item.displayObject.setZoomBucket(this._currentBucket);
+    }
+  }
+
+  /** Refresh all text/sticky items (used on initial setup). */
+  private _refreshAll(): void {
+    const bucket = this._currentBucket;
+    for (const item of this._scene.getAllItems()) {
+      if (item.type !== 'text' && item.type !== 'sticky') continue;
+      if (item.displayObject instanceof TextSprite) {
+        item.displayObject.setZoomBucket(bucket);
+      } else if (item.displayObject instanceof StickySprite) {
+        item.displayObject.setZoomBucket(bucket);
+      }
+    }
+  }
+
+  /** Refresh visible text/sticky items at current bucket (on zoom change). */
+  private _refreshVisible(): void {
+    const bucket = this._currentBucket;
+    const items = this._scene.getAllItems();
+    const vb = this._getViewportWorldBounds();
+
+    for (const item of items) {
+      if (item.type !== 'text' && item.type !== 'sticky') continue;
+      // Skip off-screen items
+      if (vb && !this._intersects(item, vb)) continue;
+
+      if (item.displayObject instanceof TextSprite) {
+        item.displayObject.setZoomBucket(bucket);
+      } else if (item.displayObject instanceof StickySprite) {
+        item.displayObject.setZoomBucket(bucket);
+      }
+    }
+  }
+
+  /** Get viewport bounds in world coordinates. */
+  private _getViewportWorldBounds(): { x: number; y: number; w: number; h: number } | null {
+    const vp = this._viewport;
+    if (!vp.screenWidth || !vp.screenHeight) return null;
+    const tl = vp.toWorld(0, 0);
+    const br = vp.toWorld(vp.screenWidth, vp.screenHeight);
+    return { x: tl.x, y: tl.y, w: br.x - tl.x, h: br.y - tl.y };
+  }
+
+  /** Check if item intersects viewport bounds (with margin). Uses world bounds for group children. */
+  private _intersects(item: SceneItem, vb: { x: number; y: number; w: number; h: number }): boolean {
+    const margin = 200; // world-space margin to include near-screen items
+    const { x: ix, y: iy, w: iw, h: ih } = getItemWorldBounds(item);
+    return (
+      ix + iw >= vb.x - margin &&
+      iy + ih >= vb.y - margin &&
+      ix <= vb.x + vb.w + margin &&
+      iy <= vb.y + vb.h + margin
+    );
+  }
+
+  /** Current bucket value (for initial setup of new items). */
+  get currentBucket(): number {
+    return this._currentBucket;
+  }
+
+  destroy(): void {
+    // nothing to clean up currently
+  }
+}