fix: improve text sharpness on zoom with tighter buckets and reliable init

Fix text staying blurry on zoom by ensuring the first setZoomBucket call
always applies (PixiJS v8 defaults to auto-resolution which skipped the
initial bucket=1 assignment), listening on both 'moved' and 'zoomed'
viewport events, and tightening bucket thresholds from 6 to 9 levels.

frontend/src/canvas/sprites/TextCore.ts

@@ -35,6 +35,7 @@ export class TextCore {
   private _lastWordWrapWidth: number;
   private _lastLineHeightMultiplier: number;
   private _zoomBucket = 1;
+  private _zoomBucketApplied = false;
 
   get measuredWidth(): number {
     return this.pixiText.width;
@@ -81,8 +82,11 @@ export class TextCore {
    * Only re-rasterizes when the bucket actually changes.
    */
   setZoomBucket(bucket: number): void {
-    if (bucket === this._zoomBucket) return;
+    // Always apply on first call (resolution starts as null/auto in PixiJS v8).
+    // After that, skip if bucket hasn't changed.
+    if (this._zoomBucketApplied && bucket === this._zoomBucket) return;
     this._zoomBucket = bucket;
+    this._zoomBucketApplied = true;
     this.pixiText.resolution = bucket;
   }