fix: sidebar — x-data vide masquait appState() + header Notion-style + null-guards

- Sidebar avait x-data vide qui créait un scope isolé → vars (workspaceMenuOpen,
  toggleWorkspaceMenu, sidebarCollapsed) introuvables → rien ne fonctionnait
- Suppression x-data → héritage du scope appState() parent
- Header restylé Notion: .sidebar-workspace-header avec border-radius + hover bg
- Mouse hover (mouseenter/mouseleave) active wsHeaderHover → fond plus clair
- Null-guards: folderStack[-1] sécurisé, ctxMenu.node && ctxMenu.node.is_folder

app/templates/base.html

@@ -58,33 +58,30 @@
   <aside class="sidebar"
          :class="{ collapsed: sidebarCollapsed, 'mobile-open': mobileSidebarOpen }"
          id="sidebar"
-         x-data x-init="$nextTick(() => { if(typeof initTreeSortable==='function') initTreeSortable(); })">
-    <!-- Workspace header + mobile back -->
-    <div style="display:flex;align-items:center;gap:4px;">
-      <button class="sidebar-back-btn" @click="mobileSidebarOpen = false; sidebarCollapsed = true;" x-show="mobileSidebarOpen" title="Fermer le menu">
-        <svg width="20" height="20" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><line x1="18" y1="6" x2="6" y2="18"/><line x1="6" y1="6" x2="18" y2="18"/></svg>
-      </button>
-      <div class="sidebar-workspace" @click="toggleWorkspaceMenu" style="flex:1;">
-        <div class="workspace-info">
-          <div class="workspace-avatar">{{ workspaceInitial }}</div>
-          <span class="workspace-name">{{ workspaceName }}</span>
-        </div>
-        <span class="workspace-chevron" :class="{ open: workspaceMenuOpen }">▾</span>
+         x-init="$nextTick(() => { if(typeof initTreeSortable==='function') initTreeSortable(); })">
+    <!-- Workspace header styled like Notion -->
+    <div class="sidebar-workspace-header" @click="toggleWorkspaceMenu" @mouseenter="wsHeaderHover=true" @mouseleave="wsHeaderHover=false"
+         :class="{ hover: wsHeaderHover || workspaceMenuOpen }">
+      <div class="workspace-info">
+        <div class="workspace-avatar">{{ workspaceInitial }}</div>
+        <span class="workspace-name">{{ workspaceName }}</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>
+      <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 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 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>
 
@@ -523,6 +520,7 @@
       sidebarCollapsed: false,
       mobileSidebarOpen: false,
       showInvite: true,
+      wsHeaderHover: false,
       workspaceName: '{{ workspace_name }}',
       workspaceInitial: '{{ workspace_initial }}',
       currentPage: '{{ current_page }}',