app: Expo skeleton and pairing with a PBX

MB3 (PBX spec 13 §5). Expo SDK 58 + Expo Router, with the PBX repo as a
submodule for @selfphoneng/shared types (import type only).

- pair by scanning the QR from My settings → Mobile app, by typing the
  server and code, or from a selfphoneng://pair link (waits for a tap)
- the device token lives in expo-secure-store; a 401 goes back to pairing
- home shows who and which extension the phone is paired as; Sign out
  removes the phone on the server too
- README: Android APK build (and the Windows long-path fix), iPhone
  builds with ios-builder

tsconfig.json

@@ -0,0 +1,9 @@
+{
+  "extends": "expo/tsconfig.base",
+  "compilerOptions": {
+    "strict": true,
+    "allowImportingTsExtensions": true,
+    "noEmit": true
+  },
+  "include": ["src", ".expo/types/**/*.ts", "expo-env.d.ts"]
+}