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

app/templates/base.html

@@ -61,7 +61,7 @@
          x-data x-init="$nextTick(() => { if(typeof initTreeSortable==='function') initTreeSortable(); })">
     <!-- Workspace header + mobile back -->
     <div style="display:flex;align-items:center;gap:4px;">
-      <button class="sidebar-back-btn" @click="mobileSidebarOpen = false; sidebarCollapsed = true;" title="Fermer le menu">
+      <button class="sidebar-back-btn" @click="mobileSidebarOpen = false; sidebarCollapsed = true;" x-show="mobileSidebarOpen" title="Fermer le menu">
         <svg width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><line x1="18" y1="6" x2="6" y2="18"/><line x1="6" y1="6" x2="18" y2="18"/></svg>
       </button>
       <div class="sidebar-workspace" @click="toggleWorkspaceMenu" style="flex:1;">
@@ -69,7 +69,22 @@
           <div class="workspace-avatar">{{ workspaceInitial }}</div>
           <span class="workspace-name">{{ workspaceName }}</span>
         </div>
-        <span class="workspace-chevron">▾</span>
+        <span class="workspace-chevron" :class="{ open: workspaceMenuOpen }">▾</span>
+      </div>
+      <!-- User menu dropdown -->
+      <div class="user-menu-dropdown" x-show="workspaceMenuOpen" @click.outside="workspaceMenuOpen=false" x-transition>
+        <div class="um-user-info">
+          <div class="um-avatar">{{ workspaceInitial }}</div>
+          <div>
+            <div class="um-name">{{ user.full_name or user.login }}</div>
+            <div class="um-plan">{{ user.email or '' }}</div>
+          </div>
+        </div>
+        <div class="um-sep"></div>
+        <a href="/settings" class="um-item" @click="workspaceMenuOpen=false">⚙️ Settings</a>
+        <a href="/workspaces" class="um-item" @click="workspaceMenuOpen=false">🔄 Switch workspace</a>
+        <div class="um-sep"></div>
+        <a href="/auth/logout" class="um-item um-danger" @click="workspaceMenuOpen=false">🚪 Log out</a>
       </div>
     </div>
 
@@ -599,13 +614,9 @@
         try { localStorage.setItem('fd_sections', JSON.stringify(this.sectionsOpen)); }
         catch(e) {}
       },
+      workspaceMenuOpen: false,
       toggleWorkspaceMenu() {
-        if (window.innerWidth <= 768) {
-          this.mobileSidebarOpen = !this.mobileSidebarOpen;
-          this.sidebarCollapsed = !this.mobileSidebarOpen;
-        } else {
-          this.sidebarCollapsed = !this.sidebarCollapsed;
-        }
+        this.workspaceMenuOpen = !this.workspaceMenuOpen;
       },
       navigateTo(url) {
         this.mobileSidebarOpen = false;