feat: restore unfinished document drafts

[VincenzoImp]

Layer 2 of the follow-up stack announced in #25, rebased onto the updated dev.

Summary

• persist unfinished document drafts atomically in the app-private, OS-managed cache on native platforms
• restore a valid current or backup draft after process death, and serialize restore/save operations
• exclude app data from Android OS backups (android:allowBackup="false") so drafts are never copied off-device
• guard navigation while processing is in flight
• add confirmations or Undo for destructive actions, including an explicit clear/keep choice after sharing
• keep web persistence an intentional no-op (in-progress pages stay memory-only, as before)
• document the retention behavior in README, SECURITY, the F-Droid description, and the feature-request template

Verification

• formatter and flutter analyze clean
• draft, recovery, navigation, destructive-action, and corruption tests pass
• complete stack: 138 tests pass
• the rebase onto current dev was conflict-free, and the resulting tree is identical to the verified one apart from the 1.3.0 release bump

Remaining follow-ups

Each will be rebased onto dev and opened here after this layer merges:

1. Bound and serialize image processing
2. Improve scanner accessibility and platform support