Global Database 0.1.10 - Implement legacy-to-global migration process and UX

Implement the Android migration experience and the gate that coordinates the transition to global mail storage. Reuse the existing database-migration activity or introduce a suitable replacement for progress, completion, failure feedback, retry, and local report export.

The migration gate controls startup, sync, workers, and other mail entry points until migration completes or fails.

Acceptance criteria

• The migration screen shows non-sensitive progress and a clear completion or failure state.
• For a POP3 account, the user may decline the archive after an explicit warning and continue at their own risk.
• Failure provides a next action, retry when safe, and local report export.
• Reports exclude mail content, addresses, attachment names, paths, credentials, and tokens.
• Startup, sync, workers, and other background mail work cannot bypass the migration gate.
• Failure before cutover leaves legacy storage authoritative.
• Tests cover interruption, retry, migration-gate behavior, redaction, and successful cutover.

Start by tracing the existing database-migration activity and the startup, sync, worker, and other mail entry points that need to respect the migration gate. Done means the migration UX covers progress, completion, failure, retry, POP3 warning, and redacted local reports, with tests for interruption, retry, gate enforcement, redaction, and successful cutover.