feat(theme-wizard): HSL Reset button + Random gets hex palette + HSL nudge

- HslNudgePanel: reusable H/S/L slider trio with built-in Reset
  (disabled when nudge is already zero)
- ColorHarmonyPicker uses HslNudgePanel
- RandomPreview rewritten: hex palette via generateRandomHexTheme,
  4-channel preview, HSL nudge with Reset, Regenerate rerolls the
  underlying random hex seed (HSL nudges persist across rerolls)
- generateRandomHexTheme: random hex base + random harmony, runs
  through hex-mode generator, applies optional HSL nudge