feat(theme-wizard): no-scroll layout, palette at bottom, drop labels

- Generated Palette moved below the two-column body (full width,
  above the action button) — reads as the 'final' result
- Drop 'Name Your Theme' + 'Generate from Color' headings (the
  controls are self-explanatory and the labels were eating space)
- Whole panel now fills available height: harmony chips at top
  (shrink-0), two-col body flex-1, palette + action shrink-0. The
  base color swatch grid grows with the column via grid-auto-rows:
  1fr — no scroll on a typical screen
- RandomPreview matches the new shape (sliders fill, palette below,
  Regenerate at bottom)