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/user_store.go

@@ -0,0 +1,73 @@
+package store
+
+import (
+	"database/sql"
+	"fmt"
+	"krafttrainer/internal/model"
+)
+
+// ListUsers gibt alle Nutzer zurück.
+func (s *Store) ListUsers() ([]model.User, error) {
+	rows, err := s.db.Query(`SELECT id, name, created_at FROM users ORDER BY created_at`)
+	if err != nil {
+		return nil, fmt.Errorf("Nutzer abfragen: %w", err)
+	}
+	defer rows.Close()
+
+	var users []model.User
+	for rows.Next() {
+		var u model.User
+		if err := rows.Scan(&u.ID, &u.Name, &u.CreatedAt); err != nil {
+			return nil, fmt.Errorf("Nutzer scannen: %w", err)
+		}
+		users = append(users, u)
+	}
+	if users == nil {
+		users = []model.User{}
+	}
+	return users, rows.Err()
+}
+
+// CreateUser legt einen neuen Nutzer an.
+func (s *Store) CreateUser(name string) (*model.User, error) {
+	result, err := s.db.Exec(`INSERT INTO users (name) VALUES (?)`, name)
+	if err != nil {
+		return nil, fmt.Errorf("Nutzer erstellen: %w", err)
+	}
+	id, _ := result.LastInsertId()
+	return s.getUser(id)
+}
+
+// DeleteUser löscht einen Nutzer, sofern noch mindestens ein weiterer existiert.
+func (s *Store) DeleteUser(id int64) error {
+	var count int
+	if err := s.db.QueryRow(`SELECT COUNT(*) FROM users`).Scan(&count); err != nil {
+		return fmt.Errorf("Nutzeranzahl prüfen: %w", err)
+	}
+	if count <= 1 {
+		return fmt.Errorf("LAST_USER: letzter Nutzer kann nicht gelöscht werden")
+	}
+
+	result, err := s.db.Exec(`DELETE FROM users WHERE id = ?`, id)
+	if err != nil {
+		return fmt.Errorf("Nutzer löschen: %w", err)
+	}
+	rows, _ := result.RowsAffected()
+	if rows == 0 {
+		return sql.ErrNoRows
+	}
+	return nil
+}
+
+func (s *Store) getUser(id int64) (*model.User, error) {
+	var u model.User
+	err := s.db.QueryRow(`SELECT id, name, created_at FROM users WHERE id = ?`, id).
+		Scan(&u.ID, &u.Name, &u.CreatedAt)
+	if err == sql.ErrNoRows {
+		return nil, nil
+	}
+	if err != nil {
+		return nil, fmt.Errorf("Nutzer abfragen: %w", err)
+	}
+	return &u, nil
+}