Bloat: hex helpers in utils.js share validate→parse pattern

[silverbucket]

`src/lib/utils.js:129-153` — `hexToRgb`, `hexToRgba`, `darkenHex` each repeat the regex validation and per-channel parse. The recursive fallback (`return hexToRgb(DEFAULT_DIE_COLOR)`) makes them harder to read than they need to be.

Fix: extract `parseHex(hex) → [r,g,b] | null`, then write all three formatters as one-liners. Removes ~15 lines and the recursion.

(From comprehensive review, finding B2.)

[silverbucket]

Triage

Still applicable. `hexToRgb` / `hexToRgba` / `darkenHex` at `src/lib/utils.js:133-156` each repeat the regex validation and reach for `parseHexChannel` per channel; the recursive fallback is unchanged.

Importance: 3 (low). Pure DRY refactor, no behaviour change. A `parseHex(hex) → [r, g, b] | null` extractor would shrink the file by ~15 lines and remove the recursion. Safe to defer until the next time these helpers are touched.