feat: complete user config management

- Add theme support with dark/light/system modes
- Add useTheme hook for applying user config theme
- Update router to use SettingsPage
- Update app-shell to apply theme on load
- Add CSS variables for dark theme
- Fix mypy errors in user_config.py
- Quality gates pass: ruff, mypy, typecheck, lint, build

openspec/changes/user-config-management/tasks.md

@@ -0,0 +1,20 @@
+## 1. Backend User Config API
+
+- [ ] 1.1 Create UserConfig SQLAlchemy model with JSONB config column
+- [ ] 1.2 Create Alembic migration for UserConfig table
+- [ ] 1.3 Add GET /users/me/config endpoint
+- [ ] 1.4 Add PATCH /users/me/config endpoint with merge semantics
+- [ ] 1.5 Add backend tests
+
+## 2. Frontend Settings
+
+- [ ] 2.1 Create settings API client (apps/web/src/api/settings.ts)
+- [ ] 2.2 Create settings page with theme selector and git identity fields
+- [ ] 2.3 Apply theme preference on app load in AppShell
+- [ ] 2.4 Add /settings route to router
+
+## 3. Verification
+
+- [ ] 3.1 Run backend checks (ruff, mypy, pytest)
+- [ ] 3.2 Run frontend checks (typecheck, lint, build)
+- [ ] 3.3 Verify docker-compose config