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/migrations/004_add_users.up.sql

@@ -0,0 +1,12 @@
+CREATE TABLE users (
+    id         INTEGER PRIMARY KEY AUTOINCREMENT,
+    name       TEXT NOT NULL CHECK(length(name) >= 1 AND length(name) <= 50),
+    created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
+-- Standardnutzer für bestehende Daten
+INSERT INTO users (name) VALUES ('Standard');
+
+ALTER TABLE exercises     ADD COLUMN user_id INTEGER NOT NULL DEFAULT 1;
+ALTER TABLE training_sets ADD COLUMN user_id INTEGER NOT NULL DEFAULT 1;
+ALTER TABLE sessions      ADD COLUMN user_id INTEGER NOT NULL DEFAULT 1;