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/model/user.go

@@ -0,0 +1,31 @@
+package model
+
+import (
+	"errors"
+	"strings"
+	"time"
+)
+
+// User repräsentiert einen Nutzer der Applikation.
+type User struct {
+	ID        int64     `json:"id"`
+	Name      string    `json:"name"`
+	CreatedAt time.Time `json:"created_at"`
+}
+
+// CreateUserRequest enthält die Felder zum Anlegen eines Nutzers.
+type CreateUserRequest struct {
+	Name string `json:"name"`
+}
+
+// Validate prüft den Request.
+func (r *CreateUserRequest) Validate() error {
+	r.Name = strings.TrimSpace(r.Name)
+	if len(r.Name) == 0 {
+		return errors.New("Name darf nicht leer sein")
+	}
+	if len(r.Name) > 50 {
+		return errors.New("Name darf maximal 50 Zeichen lang sein")
+	}
+	return nil
+}