feat(mobile): app-lock / lock screen with biometric unlock

[Lazyartist1]

Summary

Adds the app-lock / lock screen (backlog item 28) — the wallet locks after inactivity or when backgrounded, and a device biometric is required to return. Ports the web wallet's inactivity-lock behaviour to native AppState + expo-local-authentication, so a lost or borrowed phone doesn't expose funds.

Changes

• lib/idleLock.ts — framework-agnostic countdown (native port of the web lib/idle-lock.ts). Pure, with an injectable clock; keeps the 5 / 15 / 30 / never options and the "defer while a transaction is in flight" behaviour.
• hooks/useInactivityLock.ts — wires the countdown to AppState and expo-router: background locks immediately, foreground restarts the idle countdown, and either trigger routes to /lock. Re-arms off the current route so it never fights the lock screen it just pushed.
• app/lock.tsx — lock screen. Unlock requires a real biometric via LocalAuthentication.authenticateAsync (Face ID / fingerprint, with device-passcode fallback); on success it returns to the dashboard.
• app/_layout.tsx — mounts the lock gate once at the root, alongside the existing connectivity gate.
• Adds the expo-local-authentication dependency (listed in the issue).

Acceptance

• App locks after an inactivity timeout
• App locks when backgrounded
• Face ID / fingerprint unlocks (device-passcode fallback)

Testing

• tsc --noEmit clean
• jest lib/__tests__/idleLock.test.ts — 6/6 passing (timeout fires, reset postpones, stop cancels, never-disabled, defer-while-busy, ms conversion)

Closes #456

[vercel]

@Lazyartist1 is attempting to deploy a commit to the miracle656's projects Team on Vercel.

A member of the Team first needs to authorize it.

[drips-wave]

@Lazyartist1 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

Learn more about application limits

[Miracle656]

Heads-up before this can land: main already ships an app-lock feature — app/lock.tsx, lib/appLock.ts, and app/settings/security.tsx (lock timeout + biometric unlock, from #551) — and this adds a parallel implementation (lib/idleLock.ts, its own app/lock.tsx, useInactivityLock), which is why it's conflicting. Could you rebase onto current main and reconcile with the existing app-lock (enhance lib/appLock.ts / the security settings rather than adding a second lock stack)? Happy to re-review once it's rebased.