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>

backend/internal/store/export_store.go

@@ -0,0 +1,40 @@
+package store
+
+import (
+	"fmt"
+	"time"
+)
+
+// ExportRow repräsentiert eine Zeile im Trainingsexport.
+type ExportRow struct {
+	SessionStarted time.Time
+	ExerciseName   string
+	SetNumber      int
+	WeightKg       float64
+	Reps           int
+	Note           string
+}
+
+// ExportLogs gibt alle Logs abgeschlossener Sessions eines Nutzers zurück (für AI-Export).
+func (s *Store) ExportLogs(userID int64) ([]ExportRow, error) {
+	rows, err := s.db.Query(`
+		SELECT s.started_at, sl.exercise_name, sl.set_number, sl.weight_kg, sl.reps, sl.note
+		FROM session_logs sl
+		JOIN sessions s ON s.id = sl.session_id
+		WHERE s.ended_at IS NOT NULL AND s.user_id = ?
+		ORDER BY s.started_at, sl.exercise_name, sl.set_number`, userID)
+	if err != nil {
+		return nil, fmt.Errorf("Export abfragen: %w", err)
+	}
+	defer rows.Close()
+
+	var result []ExportRow
+	for rows.Next() {
+		var row ExportRow
+		if err := rows.Scan(&row.SessionStarted, &row.ExerciseName, &row.SetNumber, &row.WeightKg, &row.Reps, &row.Note); err != nil {
+			return nil, fmt.Errorf("Export scannen: %w", err)
+		}
+		result = append(result, row)
+	}
+	return result, rows.Err()
+}