Add opt-in local server for a companion mobile app

[mahdi-salmanzade]

What

Adds an opt-in, read-only local HTTP server so a companion mobile app can display your current usage over the local network. This implements the "Mobile App" settings tab, replacing the previous coming-soon placeholder with a real pairing screen.

A matching open-source Expo app lives here: https://github.com/mahdi-salmanzade/claude-usage-mobile

How it works

Claude API ──sessionKey──▶ this app (LocalServerService)
                                │  GET /v1/usage  (Bearer token, read-only)
                                ▼
                          phone (companion app) over LAN / Tailscale

The server simply serves the already-computed DataStore.shared.loadUsage() snapshot as JSON. The phone never authenticates to Claude — the session key never leaves the Mac.

Security model

• Disabled by default — the user must explicitly enable it in Settings → Mobile App.
• Token-gated — every request requires Authorization: Bearer <token>; the token is generated locally and only shared via the pairing QR code. Regenerating it un-pairs existing devices.
• Read-only — single data endpoint (GET /v1/usage) plus GET /v1/ping; unknown paths → 404, bad/missing token → 401.

Changes

• LocalServerService.swift (new) — NWListener-based HTTP server; token auth; ISO-8601 JSON envelope around ClaudeUsage; primary-LAN-address helper.
• SharedDataStore.swift — persistence for enabled/port and secure token generation.
• AppDelegate.swift — start on launch if enabled, stop on terminate.
• MobileAppView.swift — real pairing screen (enable toggle, QR code with {host,port,token}, copy/regenerate token, security note).

Testing

• Builds on Xcode 26.5.
• Verified the endpoint with curl: 401 on missing/bad token, 404 on unknown path, and a correct ClaudeUsage JSON envelope on a valid request.
• Verified end-to-end against the Expo companion app (fetch + parse + render).

Notes / follow-ups

• New MobileAppView strings are currently English literals — happy to wire them into the localization system if you'd like before merging.
• Enabling the server may trigger the macOS firewall "accept incoming connections" prompt on first run.