Remediate all P0-S priority findings from cross-platform architecture audit:
- Harden token storage with EncryptedSharedPreferences (Android) and Keychain (iOS)
- Add SSL pinning and certificate validation to API clients
- Fix subscription cache race conditions and add thread-safe access
- Add input validation for document uploads and file type restrictions
- Refactor DocumentApi to use proper multipart upload flow
- Add rate limiting awareness and retry logic to API layer
- Harden subscription tier enforcement in SubscriptionHelper
- Add biometric prompt for sensitive actions (Login, Onboarding)
- Fix notification permission handling and device registration
- Add UI test infrastructure (page objects, fixtures, smoke tests)
- Add CI workflow for mobile builds
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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