feat(themes): add theme picker with swatch previews and per-theme fonts

PART 1: Add missing CSS variables to all ui objects
- Add --bg-inset, --bg-muted for component backgrounds
- Add --success, --warning as aliases for --status-success/warning
- Add --warning-muted for warning background states
- Each theme now has 32 CSS variables

PART 2: Per-theme font support
- Move --font-sans and --font-mono from :root to ui objects
- Add fontImports array to theme config (for future custom fonts)
- applyThemeUI() now manages <link> tags for font imports
- Existing themes use empty fontImports (system fonts already loaded)

PART 3: Swatch preview colors
- Add swatch array (3 hex colors) to each theme for visual preview
- light: warm tan, sage green, khaki
- dark: dark brown, sage green, tan
- clean: light gray, Google blue, Google green
- themeList() now returns swatch in result shape

PART 4: Theme picker UI
- New ThemePicker component replaces icon toggle in header
- Palette icon trigger opens popover below
- Shows all themes as circular swatches (conic gradient)
- Active theme has accent ring indicator
- Click swatch to apply theme, closes popover
- Click outside or Escape closes popover
- Styled with current theme CSS variables

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/index.css

@@ -11,11 +11,8 @@
    ═══════════════════════════════════════════════════════ */
 
 :root {
-  /* ── Typography ── */
-  --font-sans: 'Inter', system-ui, -apple-system, sans-serif;
-  --font-mono: 'JetBrains Mono', ui-monospace, monospace;
-
   /* ── Type scale ── */
+  /* Font families (--font-sans, --font-mono) are now in theme ui objects */
   --text-xs: 0.6875rem;   /* 11px */
   --text-sm: 0.8125rem;   /* 13px */
   --text-base: 0.875rem;  /* 14px */