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/style.css

@@ -2145,6 +2145,76 @@ select {
   font-size: 0.6rem;
   font-weight: 700;
 }
+/* Theme picker grid */
+.theme-grid {
+  display: grid;
+  grid-template-columns: repeat(auto-fill, minmax(160px, 1fr));
+  gap: 10px;
+  margin-top: 12px;
+}
+.theme-card {
+  background: var(--bg-secondary);
+  border: 2px solid var(--border);
+  border-radius: 8px;
+  padding: 10px;
+  cursor: pointer;
+  transition: border-color 0.2s, transform 0.15s;
+}
+.theme-card:hover { border-color: var(--accent); transform: translateY(-1px); }
+.theme-card.active { border-color: var(--accent-green); }
+.theme-card-preview {
+  height: 40px;
+  border-radius: 4px;
+  margin-bottom: 8px;
+  overflow: hidden;
+  display: flex;
+  flex-direction: column;
+}
+.theme-card-preview-bar {
+  height: 14px;
+}
+.theme-card-preview-body {
+  flex: 1;
+  display: flex;
+  align-items: center;
+  padding: 0 6px;
+}
+.theme-card-preview-btn {
+  width: 50%;
+  height: 10px;
+  border-radius: 3px;
+}
+.theme-card-name {
+  font-size: 0.75rem;
+  font-weight: 600;
+  color: var(--text-primary);
+  margin-bottom: 2px;
+}
+.theme-card-desc {
+  font-size: 0.65rem;
+  color: var(--text-muted);
+}
+.theme-mode-toggle {
+  display: flex;
+  gap: 0;
+  margin-top: 16px;
+  border: 1px solid var(--border);
+  border-radius: 6px;
+  overflow: hidden;
+  width: fit-content;
+}
+.theme-mode-btn {
+  padding: 4px 14px;
+  font-size: 0.7rem;
+  border: none;
+  background: transparent;
+  color: var(--text-secondary);
+  cursor: pointer;
+}
+.theme-mode-btn.active {
+  background: var(--accent);
+  color: #fff;
+}
 .saved-search-vault {
   font-size: 0.65rem;
   color: var(--text-muted);