feat: enhance UI, fix backend routes, and add E2E quality gates

- UI: Polish PlanView toggles, buttons, and fix missing icons (running/activity).
- Backend: Fix route shadowing in main.py, add startup route check, and improve API test coverage.
- Tests: Add Playwright E2E smoke test, eslint-plugin-import, and integrate all into 'make check'.

Makefile

@@ -36,7 +36,11 @@ build:
 	@echo "Building frontend..."
 	cd $(FRONTEND_DIR) && npm run build
 
-check: lint coverage
+test-e2e:
+	@echo "Running E2E Smoke Tests..."
+	cd $(FRONTEND_DIR) && npm run test:e2e
+
+check: lint coverage build test-e2e
 	@echo "Pipeline check passed!"
 
 run: