feat: Sprint 26 — pluggable UI themes (dark, light, solarized, monokai, nord)

Five built-in themes with instant switching, persistent preference,
and zero-flicker loading. Custom themes are pure CSS additions.

Theme system:
- CSS variable overrides via :root[data-theme="name"] blocks
- Flicker prevention: inline <script> reads localStorage before
  stylesheet parses, preventing dark-flash on light-mode users
- Server-side persistence via settings.json (theme field)
- Boot.js syncs server preference to DOM + localStorage

Built-in themes:
- Dark (default): deep navy/indigo, muted blue accents
- Light: clean white/gray, high contrast, scrollbar overrides
- Solarized Dark: teal background, warm accents
- Monokai: warm dark, green/pink accents
- Nord: arctic blue-gray, calm and minimal

UI integration:
- Settings panel: theme dropdown with instant live preview
- /theme slash command: /theme dark|light|solarized|monokai|nord
- No enum constraint on theme setting — custom themes just work

Documentation:
- THEMES.md: how to switch themes, create custom themes, contribute

8 new tests. All 408 tests pass.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

static/boot.js

@@ -308,7 +308,7 @@ document.querySelectorAll('.suggestion').forEach(btn=>{
 
 (async()=>{
   // Load send key preference
-  try{const s=await api('/api/settings');window._sendKey=s.send_key||'enter';window._showTokenUsage=!!s.show_token_usage;window._showCliSessions=!!s.show_cli_sessions;}catch(e){window._sendKey='enter';window._showTokenUsage=false;window._showCliSessions=false;}
+  try{const s=await api('/api/settings');window._sendKey=s.send_key||'enter';window._showTokenUsage=!!s.show_token_usage;window._showCliSessions=!!s.show_cli_sessions;const _theme=s.theme||'dark';document.documentElement.dataset.theme=_theme;localStorage.setItem('hermes-theme',_theme);}catch(e){window._sendKey='enter';window._showTokenUsage=false;window._showCliSessions=false;}
   // Fetch active profile
   try{const p=await api('/api/profile/active');S.activeProfile=p.name||'default';}catch(e){S.activeProfile='default';}
   // Update profile chip label immediately