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/canvas/textLimits.ts

@@ -1,9 +1,9 @@
 /**
  * textLimits — single source of truth for font-size constraints.
  *
- * Every path that creates, resizes, or mutates fontSize on text/sticky
- * items must go through these helpers. This prevents inconsistent
- * clamping when ranges are widened later.
+ * 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.
  */
 
 // Plain text: generous range for direct manipulation