feat(mobile): pre-approval flow (expected visitors)

[ankitsejwal]

Wires the pre-approval / 'expected visitor' flow into the mobile app. The backend was fully built (create/list/redeem under /visitors/pre-approvals) but had no client methods and no UI — this adds both.

What

• Client methods: createPreApproval, redeemPreApproval.
• Resident — new /pre-approve screen: enter visitor name + apartment → get a short shareable code (defaults to ONE_TIME). Linked from the home screen.
• Guard — the /gate screen gains a code input; redeeming a valid code auto-creates an ENTERED entry for the expected visitor and refreshes the list.

Verified E2E (expo-web + Chrome DevTools, live Neon)

• Resident generated a code (7F56BDFE) for 'Expected Guest' @ B-201.
• Guard entered the code on the gate → 'Expected Guest' appeared as ENTERED with a Check out action.
• Console clean; full gate green (check-types, lint, test).

Follow-ups

• QR generation + guard QR scanning (tasks #60/#61) — this ships the code-based path (#62); QR is a natural layer on top.
• Optional validity window / max-uses fields in the resident form (backend already supports validUntil + maxUses).