Adds an escape hatch for when local DB falls out of sync with server:
- New clearForResync() on EmailRepository/MailboxRepository truncates
local emails, mailboxes, pending changes and sync-state checkpoints.
EmailBodies are preserved (FK disabled during delete) so viewed content
is not re-downloaded after re-sync.
- AccountSyncManager.forceResync() orchestrates clear + loop restart.
- Edit account screen gets an "Force full sync" OutlinedButton with a
confirmation dialog; shows spinner while the operation runs.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
