fix: theme color parsing breaks on #rgb shorthand hex (#1213)

* refactor: add pure hexToRgb helper that handles #rgb shorthand

* fix: handle #rgb shorthand hex in theme color parsing

* test: hexToRgb expands shorthand and rejects invalid input

static/js/color/hex.js

@@ -0,0 +1,14 @@
+// static/js/color/hex.js
+//
+// Parse a CSS hex color into {r, g, b}. Pure — no DOM — so it can be reused
+// across modules and unit-tested under node.
+
+// Accepts "#rgb", "#rrggbb" (with or without the leading '#'). Returns null
+// for anything that isn't a valid 3- or 6-digit hex color.
+export function hexToRgb(hex) {
+  let h = String(hex || '').trim().replace(/^#/, '');
+  if (h.length === 3) h = h.split('').map((c) => c + c).join('');
+  if (!/^[0-9a-fA-F]{6}$/.test(h)) return null;
+  const n = parseInt(h, 16);
+  return { r: (n >> 16) & 255, g: (n >> 8) & 255, b: n & 255 };
+}