Add multi-user support with export feature

- New users table (migration 004) with user_id on exercises, training_sets, sessions
- User CRUD endpoints (GET/POST /api/v1/users, DELETE /api/v1/users/{id})
- All existing endpoints scoped to X-User-ID header
- CSV export endpoint (GET /api/v1/export) for completed sessions
- UserGate in PageShell: blocks app until a user is selected
- Settings page for managing users (create, switch, delete)
- BottomNav/Sidebar updated with settings navigation
- Fix: nil pointer panic in handleDeleteUser on success path
- Fix: export download now uses fetch with X-User-ID header instead of window.location.href

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/layout/BottomNav.tsx

@@ -1,4 +1,5 @@
 import { NavLink } from 'react-router-dom';
+import { useUserStore } from '../../stores/userStore';
 
 const navItems = [
   {
@@ -37,6 +38,16 @@ const navItems = [
       </svg>
     ),
   },
+  {
+    to: '/settings',
+    label: 'Einstellungen',
+    icon: (
+      <svg className="w-6 h-6" fill="none" viewBox="0 0 24 24" stroke="currentColor" strokeWidth={2}>
+        <path strokeLinecap="round" strokeLinejoin="round" d="M10.325 4.317c.426-1.756 2.924-1.756 3.35 0a1.724 1.724 0 002.573 1.066c1.543-.94 3.31.826 2.37 2.37a1.724 1.724 0 001.065 2.572c1.756.426 1.756 2.924 0 3.35a1.724 1.724 0 00-1.066 2.573c.94 1.543-.826 3.31-2.37 2.37a1.724 1.724 0 00-2.572 1.065c-.426 1.756-2.924 1.756-3.35 0a1.724 1.724 0 00-2.573-1.066c-1.543.94-3.31-.826-2.37-2.37a1.724 1.724 0 00-1.065-2.572c-1.756-.426-1.756-2.924 0-3.35a1.724 1.724 0 001.066-2.573c-.94-1.543.826-3.31 2.37-2.37.996.608 2.296.07 2.572-1.065z" />
+        <path strokeLinecap="round" strokeLinejoin="round" d="M15 12a3 3 0 11-6 0 3 3 0 016 0z" />
+      </svg>
+    ),
+  },
 ];
 
 export function BottomNav() {
@@ -64,10 +75,12 @@ export function BottomNav() {
 }
 
 export function Sidebar() {
+  const { activeUser } = useUserStore();
+
   return (
     <nav className="hidden md:flex flex-col w-56 bg-gray-900 border-r border-gray-800 min-h-screen p-4">
       <h1 className="text-xl font-bold text-blue-500 mb-8">Krafttrainer</h1>
-      <div className="flex flex-col gap-1">
+      <div className="flex flex-col gap-1 flex-1">
         {navItems.map((item) => (
           <NavLink
             key={item.to}
@@ -86,6 +99,14 @@ export function Sidebar() {
           </NavLink>
         ))}
       </div>
+      {activeUser && (
+        <div className="pt-4 border-t border-gray-800 flex items-center gap-2">
+          <div className="w-7 h-7 rounded-full bg-blue-600 flex items-center justify-center text-white text-xs font-bold flex-shrink-0">
+            {activeUser.name.charAt(0).toUpperCase()}
+          </div>
+          <span className="text-sm text-gray-300 truncate">{activeUser.name}</span>
+        </div>
+      )}
     </nav>
   );
 }