feat(backup): add verified vault backups and restore preflight

[kcng0]

Summary

• export manifest v5 backups from one stable Drift snapshot with streamed byte-length and SHA-256 verification
• preflight every manifest, path, organization reference, and payload before restore mutates the vault or database
• add cancellable progress, retryable typed errors, and localized Settings UI for export and restore
• preserve v1-v4 restore compatibility with an explicit non-checksum-verified result

Reliability boundaries

• verified means the manifest and payload bytes matched at one point in time
• it does not mean encryption, authenticated tamper resistance, or continuous backup health
• the CodeStable issue remains open for physical-device, low-storage, permission-revocation, and real payload-corruption checks

Verification

• flutter gen-l10n
• flutter pub run build_runner build
• dart format --output=none --set-exit-if-changed lib test (206 files)
• flutter analyze (0 issues)
• git diff --check
• flutter test --reporter failures-only (251 tests)
• flutter build apk --debug

No Android or iOS physical-device validation has been run yet.

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