Match sidebar bottom layout to the Wochenplanung reference (theme toggle + Abmelden in one row)

Square icon-only theme toggle sits next to the Abmelden button in a shared
row, collapsing to just the toggle when the sidebar is collapsed - same
arrangement as pm-app's Sidebar.tsx.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/frontend/components/Sidebar.tsx

@@ -200,12 +200,17 @@ export function Sidebar({ isCollapsed, onToggleCollapsed }: SidebarProps) {
             <span className="sidebar-identity-name">{user.name}</span>
           </div>
         )}
-        <ThemeToggle />
-        {user && (
-          <button type="button" className="sidebar-logout" onClick={() => void logout()}>
-            {isCollapsed ? "⏻" : "Abmelden"}
-          </button>
-        )}
+        {/* Toggle and Abmelden share one row: the toggle stays a fixed-size
+            square, Abmelden takes the rest of the width. Collapsed, only
+            the square toggle remains (centered). */}
+        <div className="sidebar-bottom-row">
+          <ThemeToggle />
+          {user && !isCollapsed && (
+            <button type="button" className="sidebar-logout" onClick={() => void logout()}>
+              Abmelden
+            </button>
+          )}
+        </div>
       </div>
     </aside>
   );