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

app.json

@@ -0,0 +1,38 @@
+{
+  "expo": {
+    "name": "SelfPhoneNG",
+    "slug": "selfphoneng",
+    "version": "0.0.1",
+    "scheme": "selfphoneng",
+    "orientation": "portrait",
+    "icon": "./assets/icon.png",
+    "userInterfaceStyle": "light",
+    "ios": {
+      "bundleIdentifier": "bet.dou.selfphoneng",
+      "supportsTablet": true
+    },
+    "android": {
+      "package": "bet.dou.selfphoneng",
+      "adaptiveIcon": {
+        "backgroundColor": "#E6F4FE",
+        "foregroundImage": "./assets/android-icon-foreground.png",
+        "backgroundImage": "./assets/android-icon-background.png",
+        "monochromeImage": "./assets/android-icon-monochrome.png"
+      },
+      "predictiveBackGestureEnabled": false
+    },
+    "plugins": [
+      "expo-router",
+      "expo-secure-store",
+      [
+        "expo-camera",
+        {
+          "cameraPermission": "$(PRODUCT_NAME) uses the camera to scan the pairing QR code.",
+          "microphonePermission": false,
+          "recordAudioAndroid": false,
+          "barcodeScannerEnabled": true
+        }
+      ]
+    ]
+  }
+}