feat(startup): replace legacy migration with a backup gate, drop hive

[ethicnology]

hive was only kept alive by the legacy storage migrations: the v0.1–v0.4 → v5
chain (004_legacy + 005_hive_to_sqlite) and the legacy_seed_view recovery
feature. Those paths serve BULL builds from 2023–2024 (Android only); the app is
at v6.13.

Simply deleting the migration would strand any user still on those builds: the
app would start empty with no guidance.

What changes

Pre-v5 installs are now gated behind a backup screen instead of being migrated
(commit 1):

• CheckLegacyInstallUsecase detects the legacy version marker in secure
  storage (Android only — other platforms never shipped a pre-v5 build, and the
  guard avoids the iOS pre-first-unlock keychain error).
• GetLegacySeedsUsecase enumerates legacy seeds without Hive: the old app
  always stored the seed material itself (keyed by fingerprint) in secure
  storage; Hive only held the wallet index.
• LegacyBackupScreen blocks startup: it shows each recovery phrase (and any
  non-empty passphrases — omitting them would strand funds), makes the user
  confirm they wrote everything down, then instructs them to delete and
  reinstall the app and recover. If the marker says legacy but no seed is
  readable, it points to support instead of suggesting a delete.
• The migration code becomes unreachable at this commit and is removed in the
  next two.

Then the legacy path is deleted (commits 2–3): the legacy_seed_view
feature, the 004_legacy + 005_hive_to_sqlite migrations,
RequiresMigrationUsecase, all DI wiring, and the hive dependency itself
(−3 700 lines).

Security notes (key material — please review carefully)

• The new screen displays recovery phrases. It follows the repo's sealed-UI
  rules: the bloc state carries no seed (legacyBackupRequired is a plain
  marker); the widget reads the seeds internally and never returns them;
  no_screenshot is enabled while shown; the words and passphrases are excluded
  from the semantics/accessibility tree; nothing is logged.
• Reinstalling wipes app data (including the old Hive files) on Android, which
  is why the flow instructs a reinstall rather than an in-app reset.

What deliberately does NOT change

• Users already migrated (v5+) are unaffected — the gate only triggers on the
  pre-v5 version marker.
• hive leaves the lockfile entirely; nothing else depended on it.
• No change to the current (post-v5) schema migrations (schema_*_to_*.dart).
• legacy_seed_view was never in the FEATURES.md graph, so no graph update is
  needed.

Commit reading order

1. feat(startup): gate legacy installs behind a seed-backup screen — the
   behaviour change: detection, seed enumeration, sealed screen, l10n, tests.
2. refactor(settings): drop the legacy_seed_view feature — feature + settings
   wiring + dead l10n keys.
3. refactor(storage): drop the legacy v4/v5 hive migration path — migrations,
   DI wiring, hive dependency.

Test plan

Automated (in this PR):

• make analyze — no issues (matches CI: --fatal-warnings --fatal-infos)
• make unit-test — all green, including 10 new tests:
  CheckLegacyInstallUsecase (marker prefixes, non-Android guard, current
  version) and GetLegacySeedsUsecase (valid seed parsing, passphrase
  filtering, non-seed entries skipped, empty store)
• fvm flutter pub get --enforce-lockfile resolves without hive

Manual QA:

• Fresh install / already-migrated install: startup is unchanged (no gate)
• Legacy install (version marker 0.1–0.4 in secure storage): the backup
  gate appears, shows the recovery phrase(s) + passphrase(s), screenshots are
  blocked, and the confirm checkbox reveals the reinstall instructions
• Settings → Bitcoin: the "Legacy seeds" entry is gone

[ethicnology]

@claude review this please

[ethicnology]

@wired-pasteque can you try the legacy apk https://github.com/SatoshiPortal/bullbitcoin-mobile/actions/runs/30940542068 migration to the current branch https://github.com/SatoshiPortal/bullbitcoin-mobile/actions/runs/30941630233 ?

[wired-pasteque]

@ethicnology, tested on my side - did the full loop, recovery included - everything is working as expected:

Installed the legacy APK, set up my wallet, then installed the new build over the top. The notice screen appeared as expected with the steps on what to do (see the screenshot), legacy seeds line in Settings still showed the seed at that point.

After reinstalling the app and recovering - balance still correct, nothing lost, and the legacy seeds line in Settings is now gone. Fresh and already-migrated installs start normally with no gate. Working as expected on my end.

[ethicnology]

Do you ACK the concept @i5hi @BullishNode @thibistaken ?

Getting rid of the unmaintained dependency hive by dropping the migration from the archaic 0.4.X to >5.X.X (sqlite) by forcing the user to make a backup and re-install the app to recover. This is the same kind of solution we offered for the forced migration to flutter_secure_storage version 10

[i5hi]

conceptACK