feat: #65 systeme de themes — 15 themes x dark/light

- themes.js: 15 palettes (Defaut ObsiGate, GitHub, Linear, Spotify, Notion,
  Discord, Dracula, One Dark, Tokyo Night, Nord, Night Owl, Anthracite,
  Cyberpunk, Navy Elegance, Tons Terreux)
- Chaque theme a un mode sombre ET clair
- UI: grille de themes dans Configuration → Themes
- Toggle Sombre/Clair par theme
- Persistance localStorage (obsigate-theme + obsigate-theme-mode)
- Preview visuel: barre de couleur + accent bouton
- initThemes() appele au startup dans sync.js
- Defaut ObsiGate conserve le theme actuel

frontend/js/sync.js

@@ -6,6 +6,7 @@ import { loadVaults, loadTags, refreshTagsForContext, refreshSidebarTreePreservi
 import { loadRecentFiles } from './config.js';
 import { openFile } from './viewer.js';
 import { initOffline, syncFileIndexFromServer } from './offline.js';
+import { initThemes } from './themes.js';
 
 // ---------------------------------------------------------------------------
 // SSE Client — IndexUpdateManager
@@ -552,6 +553,7 @@ export { initSyncStatus, initDashboardTabs };
 export function init() {
   registerServiceWorker();
   initOffline();
+  initThemes();
   // Sync file index after a short delay (let vaults load first)
   setTimeout(() => syncFileIndexFromServer(), 3000);
   initDashboardTabs();