[tribhuvneshsharma]

Closes #11

What this PR does

Adds four end-to-end test scenarios in e2e/run_e2e.py covering cancellation flows and the no-drivers search timeout, along with backend configuration:

1. Rider cancels while searching: Rider sees "Ride cancelled" and can book another ride.
2. Rider cancels after driver assigned: Driver is notified on their trip screen and returned to home screen.
3. Driver cancels after accepting: Driver returns home, and rider sees the search restart ("Finding you a driver").
4. No drivers nearby: Search ends with "No drivers available right now" after the timeout.

Also updates Settings in backend/app/config.py so that search_timeout_seconds reads from the SEARCH_TIMEOUT_SECONDS environment variable (defaulting to 180), and adds a unit test in backend/tests/test_api.py.

Checklist

• Assigned the issue before starting
• Each scenario starts from a fresh login session (reset_session)
• Handled browser confirmation/alert dialogs (page.on("dialog", ...))
• Backend tests pass (pytest backend/tests/test_api.py)