feat(theming): in-UI theme editor (2.10.0)

Recolour the whole UI from Settings — 12 palette colour pickers with
live preview, presets (Ember/Frost/Verdant/Amethyst), and reset to the
default Blackflame. Overrides persist in app_settings (key 'theme') via
a hex-validated /api/theme route and apply to :root on boot.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/app.js

@@ -11,6 +11,7 @@ import { emit, state } from './state.js';
 import { el, mount } from './dom.js';
 import { attachDropzone } from './components/dropzone.js';
 import { initChrome } from './components/chrome.js';
+import { loadTheme } from './theme.js';
 
 const VIEWS = {
   home:            () => import('./views/home.js'),
@@ -80,6 +81,7 @@ async function init() {
     try { await api.get('/api/spaces'); }
     catch { /* api wrapper opens the modal on 401 */ }
   }
+  await loadTheme();            // apply saved palette overrides before rendering chrome
   renderTopbar(document.getElementById('topbar'));
   renderSidebar(document.getElementById('sidebar'));
   renderRightrail(document.getElementById('rightrail'));