Make tip submission idempotent against double taps

[RUKAYAT-CODER]

Background

Rapid taps can submit the same tip twice. Add a client-generated idempotency key.

Files to modify (2-3)

src/services/tipService.ts
src/lib/featureFlags/tipCanary.ts

Acceptance Criteria

Implemented across the listed files

Unit/integration tests added or updated and passing

No regression; follows project coding standards