feat: sidebar user menu Notion-style + settings page + tag colors + preview images/PDF

- Sidebar header: user menu dropdown (Settings, Switch workspace, Log out)
  avec avatar + nom + email, chevron animé, fond opaque #1E1E1E
- Nouvelle page /settings avec gestion globale des tags:
  créer tag avec couleur, changer couleur, supprimer, liste avec compteurs
- API tag management: POST/PUT/DELETE /api/settings/tags, GET /api/settings/tags/all
- Preview panel: affichage images (img tag), PDF (lien viewer), détection format
- workspace-chevron CSS: rotation 180° quand menu ouvert

static/css/app.css

@@ -162,6 +162,56 @@ input, select, textarea { font-family: inherit; color: inherit; }
   color: var(--text-primary);
 }
 
+.workspace-chevron {
+  font-size: 10px;
+  color: var(--text-dim);
+  transition: transform 0.2s ease;
+}
+.workspace-chevron.open {
+  transform: rotate(180deg);
+}
+
+/* User menu dropdown */
+.user-menu-dropdown {
+  position: absolute;
+  top: 100%;
+  left: 8px;
+  right: 8px;
+  margin-top: 4px;
+  background: #1E1E1E;
+  border: 1px solid var(--border-strong);
+  border-radius: 10px;
+  box-shadow: 0 12px 32px rgba(0,0,0,.6);
+  z-index: 400;
+  padding: 4px 0;
+  overflow: hidden;
+}
+.um-user-info {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+  padding: 10px 14px;
+}
+.um-avatar {
+  width: 28px; height: 28px;
+  background: var(--bg-tertiary);
+  border-radius: 6px;
+  display: flex; align-items: center; justify-content: center;
+  font-size: 14px; font-weight: 600; color: var(--text);
+  flex-shrink: 0;
+}
+.um-name { font-size: 14px; font-weight: 600; color: var(--text); }
+.um-plan { font-size: 12px; color: var(--text-dim); margin-top: 1px; }
+.um-sep { height: 1px; background: var(--border); margin: 3px 0; }
+.um-item {
+  display: flex; align-items: center; gap: 8px;
+  padding: 7px 14px; font-size: 13px; color: var(--text);
+  text-decoration: none; cursor: pointer; transition: background 100ms;
+}
+.um-item:hover { background: var(--bg-hover); }
+.um-danger { color: var(--danger); }
+.um-danger:hover { background: rgba(255,80,80,.1); }
+
 .sidebar-actions {
   display: flex;
   align-items: center;