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/panels.js

@@ -958,6 +958,9 @@ async function loadSettingsPanel(){
     // Send key preference
     const sendKeySel=$('settingsSendKey');
     if(sendKeySel) sendKeySel.value=settings.send_key||'enter';
+    // Theme preference
+    const themeSel=$('settingsTheme');
+    if(themeSel) themeSel.value=settings.theme||'dark';
     const showUsageCb=$('settingsShowTokenUsage');
     if(showUsageCb) showUsageCb.checked=!!settings.show_token_usage;
     const showCliCb=$('settingsShowCliSessions');
@@ -992,6 +995,7 @@ async function saveSettings(){
   if(model) body.default_model=model;
   if(workspace) body.default_workspace=workspace;
   if(sendKey) body.send_key=sendKey;
+  body.theme=($('settingsTheme')||{}).value||'dark';
   body.show_token_usage=showTokenUsage;
   body.show_cli_sessions=showCliSessions;
   body.sync_to_insights=!!($('settingsSyncInsights')||{}).checked;