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/pages/SettingsPage.tsx

@@ -0,0 +1,95 @@
+import { useEffect, useState } from 'react';
+import { useUserStore } from '../stores/userStore';
+
+export function SettingsPage() {
+  const { users, activeUser, setActiveUser, fetchUsers, createUser, deleteUser } =
+    useUserStore();
+  const [newName, setNewName] = useState('');
+  const [loading, setLoading] = useState(false);
+
+  useEffect(() => {
+    fetchUsers();
+  }, [fetchUsers]);
+
+  async function handleCreate(e: React.FormEvent) {
+    e.preventDefault();
+    const name = newName.trim();
+    if (!name) return;
+    setLoading(true);
+    const user = await createUser(name);
+    setLoading(false);
+    if (user) setNewName('');
+  }
+
+  async function handleDelete(id: number) {
+    await deleteUser(id);
+  }
+
+  return (
+    <div className="space-y-6">
+      <h1 className="text-2xl font-bold text-gray-100">Einstellungen</h1>
+
+      <section className="space-y-3">
+        <h2 className="text-lg font-semibold text-gray-200">Nutzer</h2>
+
+        <ul className="space-y-2">
+          {users.map((user) => (
+            <li
+              key={user.id}
+              className="flex items-center justify-between bg-gray-900 rounded-lg px-4 py-3"
+            >
+              <button
+                onClick={() => setActiveUser(user)}
+                className="flex items-center gap-3 flex-1 text-left"
+              >
+                <div
+                  className={`w-8 h-8 rounded-full flex items-center justify-center text-sm font-bold ${
+                    activeUser?.id === user.id
+                      ? 'bg-blue-600 text-white'
+                      : 'bg-gray-700 text-gray-300'
+                  }`}
+                >
+                  {user.name.charAt(0).toUpperCase()}
+                </div>
+                <span className="text-gray-100">{user.name}</span>
+                {activeUser?.id === user.id && (
+                  <span className="text-xs text-blue-400 font-medium">Aktiv</span>
+                )}
+              </button>
+
+              {users.length > 1 && (
+                <button
+                  onClick={() => handleDelete(user.id)}
+                  className="p-2 text-gray-500 hover:text-red-400 transition-colors min-h-[44px] min-w-[44px] flex items-center justify-center"
+                  title="Nutzer löschen"
+                >
+                  <svg xmlns="http://www.w3.org/2000/svg" className="w-4 h-4" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+                    <path strokeLinecap="round" strokeLinejoin="round" strokeWidth={2} d="M19 7l-.867 12.142A2 2 0 0116.138 21H7.862a2 2 0 01-1.995-1.858L5 7m5 4v6m4-6v6m1-10V4a1 1 0 00-1-1h-4a1 1 0 00-1 1v3M4 7h16" />
+                  </svg>
+                </button>
+              )}
+            </li>
+          ))}
+        </ul>
+
+        <form onSubmit={handleCreate} className="flex gap-2">
+          <input
+            type="text"
+            value={newName}
+            onChange={(e) => setNewName(e.target.value)}
+            placeholder="Neuer Nutzername"
+            maxLength={50}
+            className="flex-1 bg-gray-900 border border-gray-700 rounded-lg px-3 py-2 text-gray-100 placeholder-gray-500 focus:outline-none focus:border-blue-500 min-h-[44px]"
+          />
+          <button
+            type="submit"
+            disabled={loading || !newName.trim()}
+            className="px-4 py-2 bg-blue-600 hover:bg-blue-500 disabled:opacity-50 text-white rounded-lg font-medium min-h-[44px] transition-colors"
+          >
+            Hinzufügen
+          </button>
+        </form>
+      </section>
+    </div>
+  );
+}