Wire activity_id linkage to report submission

Extract activity_id from GoRouter's extra or pathParameters at the top of the screen's build method — use a dedicated RouteArgs data class to avoid stringly-typed extraction. The BLoC/Cubit should store activity_id in its initial state (passed via constructor injection), not derived inside the submit handler, so it is immutable for the lifetime of the form. For the Supabase insert, activity_id must be included as a literal column in the insert map — do not rely on a database trigger or default. If the foreign key check fails (PGRST301 or 23503), catch the PostgrestException and emit a user-friendly error event.

Avoid any logic that silently drops activity_id if it is unexpectedly null — prefer an explicit assert in debug builds.

Unit tests (flutter_test): ReportFormOrchestratorBloc emits error state when activity_id is null; submit method includes activity_id in payload when non-null. Widget tests: screen renders ErrorStateWidget when navigated to without activity_id argument; screen renders form scaffold when valid activity_id is provided. Integration test against Supabase test instance: submit with valid activity_id succeeds and record is retrievable; submit with non-existent activity_id surfaces a caught foreign key violation error. Minimum 90% branch coverage on route argument extraction and orchestrator submission path.