feat(sticky): S/M/L text size toggle, live reflow resize, min width

Sticky UX overhaul — clear separation between card resize and text size:

- S/M/L toggle (12/14/18px) in contextual toolbar for sticky text size
- TransformBox: horizontal-only resize for stickies with live text
  reflow during drag (no more stretch-then-snap)
- Hide vertical handles (tc/bc) for sticky-only selections
- Minimum sticky width of 80px enforced in both drag and bake
- TextEditor: sticky background auto-resizes as user types
- textLimits.ts comment corrected to match actual usage

Plain text unchanged: resize gesture scales fontSize via bake.
Fully backward compatible — existing stickies map to nearest preset.

frontend/src/hooks/useCanvasSetup.ts

@@ -233,7 +233,7 @@ export function useCanvasSetup(deps: CanvasSetupDeps) {
               const d = item.data as any;
               // Bake width only — text re-wraps and height auto-adjusts.
               // fontSize stays stable so the card acts like a resizable text box.
-              d.w = Math.round(d.w * absSx);
+              d.w = Math.max(80, Math.round(d.w * absSx));
               d.sx = item.data.sx > 0 ? 1 : -1;
               d.sy = item.data.sy > 0 ? 1 : -1;
               if (item.displayObject instanceof StickySprite) {