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/HistoryPage.tsx

@@ -1,15 +1,47 @@
 import { useState } from 'react';
 import { SessionList } from '../components/history/SessionList';
 import { ExerciseChart } from '../components/history/ExerciseChart';
+import { getActiveUserId } from '../stores/userStore';
 
 type Tab = 'history' | 'stats';
 
+async function downloadExport() {
+  const uid = getActiveUserId();
+  const headers: Record<string, string> = {};
+  if (uid) headers['X-User-ID'] = uid;
+
+  const res = await fetch('/api/v1/export', { headers });
+  if (!res.ok) return;
+
+  const blob = await res.blob();
+  const url = URL.createObjectURL(blob);
+  const a = document.createElement('a');
+  a.href = url;
+  a.download = `training-export-${new Date().toISOString().slice(0, 10)}.csv`;
+  document.body.appendChild(a);
+  a.click();
+  document.body.removeChild(a);
+  URL.revokeObjectURL(url);
+}
+
 export function HistoryPage() {
   const [activeTab, setActiveTab] = useState<Tab>('history');
 
   return (
     <div className="space-y-4">
-      <h1 className="text-2xl font-bold text-gray-100">Historie</h1>
+      <div className="flex items-center justify-between">
+        <h1 className="text-2xl font-bold text-gray-100">Historie</h1>
+        <button
+          onClick={downloadExport}
+          className="flex items-center gap-2 px-3 py-2 rounded-lg bg-gray-800 text-gray-300 hover:text-white hover:bg-gray-700 text-sm min-h-[44px] transition-colors"
+          title="Trainingsdaten als CSV exportieren"
+        >
+          <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="M4 16v1a3 3 0 003 3h10a3 3 0 003-3v-1m-4-4l-4 4m0 0l-4-4m4 4V4" />
+          </svg>
+          Export CSV
+        </button>
+      </div>
 
       {/* Tab Toggle */}
       <div className="flex bg-gray-900 rounded-lg p-1">