refactor(sticky): fixed-width card with S/M/L/XL/XXL font presets

Stickies are now fixed-width cards — no resize handles shown.
Text size controlled via 5 presets (10/14/18/24/32px).
On creation, zoom-aware screenToWorld snaps to nearest preset.
Height auto-adjusts from content. Plain text resize unchanged.

Removes: sticky width bake, live reflow drag, min width constraint.
Backward compatible — existing stickies map to nearest preset.

frontend/src/canvas/textLimits.ts

@@ -1,23 +1,17 @@
 /**
- * textLimits — single source of truth for font-size constraints.
+ * textLimits — font-size constraints for plain text items.
  *
  * Used by creation defaults to keep initial sizes reasonable.
  * Direct manipulation (resize-bake) intentionally bypasses these
  * clamps so users can scale text to any size.
+ *
+ * Sticky notes use discrete presets (S/M/L/XL/XXL) defined in
+ * TextFormatToolbar.tsx — not continuous clamps.
  */
 
-// Plain text: generous range for direct manipulation
 const TEXT_FONT_MIN = 6;
 const TEXT_FONT_MAX = 160;
 
-// Sticky note text: narrower range (card layout breaks at extremes)
-const STICKY_FONT_MIN = 8;
-const STICKY_FONT_MAX = 96;
-
 export function clampTextFontSize(size: number): number {
   return Math.round(Math.min(TEXT_FONT_MAX, Math.max(TEXT_FONT_MIN, size)));
 }
-
-export function clampStickyFontSize(size: number): number {
-  return Math.round(Math.min(STICKY_FONT_MAX, Math.max(STICKY_FONT_MIN, size)));
-}