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

.gitignore

@@ -0,0 +1,41 @@
+# Learn more https://docs.github.com/en/get-started/getting-started-with-git/ignoring-files
+
+# dependencies
+node_modules/
+
+# Expo
+.expo/
+dist/
+web-build/
+expo-env.d.ts
+
+# Native
+.kotlin/
+*.orig.*
+*.jks
+*.p8
+*.p12
+*.key
+*.mobileprovision
+
+# Metro
+.metro-health-check*
+
+# debug
+npm-debug.*
+yarn-debug.*
+yarn-error.*
+
+# macOS
+.DS_Store
+*.pem
+
+# local env files
+.env*.local
+
+# typescript
+*.tsbuildinfo
+
+# generated native folders
+/ios
+/android