feat(mobile): migrate Flutter app to Keycloak OIDC authentication

Summary

• Migrate Flutter mobile app from legacy auth to Keycloak OIDC (Authorization Code + PKCE)
• Restore mobile authentication after legacy endpoints removal (MR !36 (merged))
• Align mobile auth with web/backend architecture (token passthrough, JWKS validation)

Planning Artifacts

• PRD, architecture, epic breakdown, and sprint status tracking committed
• 6 epics, 22 stories, 6 retrospectives

Implementation Plan

See epics breakdown in _bmad-output/planning-artifacts/epics.md

Test plan

• Unit tests for auth logic (TokenStorage, AuthState, AuthNotifier, AuthInterceptor)
• Integration tests on Android (login, refresh, logout, error scenarios)
• CI pipeline on Android
• Manual testing per story acceptance criteria

Closes #613 (closed)