No inactivity auto-lock requiring re-authentication after backgrounding

[ndii-dev]

Beyond the general session-timeout issue for post-biometric-unlock state, there is currently no lock screen concept in this app at all — once isOnboarded is true, /tabs/home is fully accessible indefinitely with the app simply resuming in the foreground with no gate whatsoever, biometric or otherwise (there's no auth gate implemented yet at all). Implement an app-level lock screen that engages after configurable backgrounded duration, independent of whatever biometric/PIN gate related issues introduce at the auth-flow level.

Definition of done:

• Lock screen component that intercepts all app content when triggered, mounted at the _layout.tsx level
• Configurable timeout, sensible default (e.g. immediate lock on background for a wallet, not a lenient timeout)
• Verified it can't be bypassed via Android's back button or iOS gesture navigation

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This is not a starter-issue. The Definition of done above is the full
acceptance criteria, not a subset to sample from — a PR that addresses part
of it is an unfinished issue, not a smaller one. Your PR must include, in
the PR description itself:

• Root cause / design-decision rationale in your own words — not a restatement of this issue
• Every Definition of done bullet above addressed explicitly, with a one-line note on how
• Evidence the code actually runs: pasted test/build output, a screen recording or before/after screenshots for UI changes, or real (non-mocked) logs for network/contract-facing work
• New or updated tests included and shown passing (paste the output)
• Any adjacent/related behavior this issue calls out re-verified, not assumed unaffected

PRs missing these will be sent back before review, not reviewed and rejected — please do this up front.

[Ayinkx1]

Hi i would like to work on this issue please assign to me
I can do it perfectly