fix(android/security): allow device credential for the app lock

[farfromrefug]

Summary

• Android devices secured with a PIN/pattern/password but no enrolled biometric could never enable the app lock: available() resolves { any: true, biometrics: false } there, and the old check only looked at biometrics/touch/face (iOS-only fields). Availability now comes from any.
• verifyBiometric is called with pinFallback: true on Android, so the system prompt allows DEVICE_CREDENTIAL alongside BIOMETRIC_STRONG — without it the plugin also fails at keystore key generation (setUserAuthenticationRequired requires an enrolled biometric). Prompt title is now translated. iOS is untouched (it already uses LAPolicy.DeviceOwnerAuthentication).
• Enabling the lock re-checks availability and surfaces a message when no screen lock is set up; the settings error handling no longer breaks on the plugin's non-string rejections, so the toggle explains itself instead of silently reverting.
• The availability check can no longer take down service startup (it used to reject out of Promise.all in app.ts), and the startup lock check still runs when the lock is enabled.
• Patches @nativescript/biometrics@1.3.1: authenticating with the device credential brings another activity to the front, the JS peer of the plugin's AuthenticationCallback gets collected, and onAuthenticationSucceeded then runs on a fresh peer with default fields — it took the crypto branch and crashed on a null CryptoObject (TypeError: cannot read property 'getCipher' of null, uncaught on the main thread). The patch tracks the pending call in module scope, keeps a reference to the callback and guards the null crypto object. Worth sending upstream.
• Reworded biometric_lock_desc (English source only) — it advertised a "PIN code" that was never actually accepted.

Testing

eslint and yarn svelte-check clean (0 errors). No unit test possible — the service pulls the NativeScript runtime and the native plugin.

Manual Android check:

1. PIN/password-only device, no fingerprint → toggle opens the system credential prompt and enables. Reported crash on credential success is what the plugin patch addresses — needs a re-run to confirm.
2. Device with a fingerprint → prompt still accepts the fingerprint, PIN as fallback.
3. Device with no screen lock → toggle reverts with an explanation, app startup unaffected.
4. Lock + auto-lock on → background/foreground prompts again.

Refs #681