refactor: centralize font-size clamps in textLimits.ts

Single source of truth for font-size constraints (text 6–160, sticky
8–96) used by creation defaults, resize-bake, and any future mutation
paths. Replaces scattered hardcoded min/max values.

frontend/src/canvas/textLimits.ts

@@ -0,0 +1,23 @@
+/**
+ * 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.
+ */
+
+// 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)));
+}