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

README.md

@@ -1,3 +1,78 @@
-# mobile
+# SelfPhoneNG mobile
 
-Mobile apps for SelfPhoneNG
+The Android and iPhone app for [SelfPhoneNG](https://git.dou.bet/spng/selfphoneng), the
+self-hosted business phone system. One Expo (React Native) codebase for both. What it does and
+the order of work: the PBX repo's `docs/spec/13-mobile.md`.
+
+Status: pairing with a PBX (MB3). Calls, history, voicemail, contacts and settings come next.
+
+## Get the code
+
+```bash
+git clone --recurse-submodules https://git.dou.bet/spng/mobile.git
+npm install
+```
+
+`vendor/selfphoneng` is the PBX repo as a git submodule, for the shared types
+(`@selfphoneng/shared`). After the PBX changes its API, move it forward:
+`git -C vendor/selfphoneng pull origin main && git add vendor/selfphoneng`.
+
+## Develop
+
+```bash
+npm run typecheck
+npm run lint
+npx expo start          # needs a development build on the phone (the app has native modules)
+```
+
+## Pair a phone
+
+On a computer, open the PBX's web page → My settings → **Mobile app** → **Pair a phone**. In the
+app, tap **Scan the QR code** (or type the server and code). The code works once, for 10 minutes.
+
+## Build and sideload
+
+### Android (on Windows, Linux or macOS)
+
+Needs JDK 17+ and the Android SDK (`ANDROID_HOME`). The first build downloads the NDK and SDK
+platforms it's missing.
+
+```bash
+npx expo prebuild --platform android
+cd android && ./gradlew assembleRelease
+```
+
+Add `-PreactNativeArchitectures=arm64-v8a` for a faster build that runs on any recent phone.
+
+On Windows the C++ build fails with `ninja: error: mkdir(...): No such file or directory`: the
+paths pass 260 characters, and the CMake 3.22 that Gradle installs has a `ninja` that can't use
+long paths. Turn on Windows long paths (`LongPathsEnabled`), install a newer CMake
+(`sdkmanager "cmake;3.31.6"`), and after each `expo prebuild` point the build at it in
+`android/local.properties`:
+
+```properties
+sdk.dir=C:/Users/<you>/AppData/Local/Android/Sdk
+cmake.dir=C:/Users/<you>/AppData/Local/Android/Sdk/cmake/3.31.6
+```
+
+The APK is `android/app/build/outputs/apk/release/app-release.apk`, signed with the debug key:
+copy it to the phone and open it (allow installing from unknown sources), or `adb install` it.
+
+### iPhone (no Mac needed)
+
+[ios-builder](https://github.com/MobAI-App/ios-builder) runs the Xcode build on a GitHub Actions
+macOS runner, so it needs this repo mirrored to GitHub.
+
+1. Download `builder-windows-amd64.exe` from its releases, rename it to `builder.exe`, put it on
+   your PATH, then `builder auth github`.
+2. In this repo: `builder init`. It adds `.github/workflows/ios-build.yml` and `builder.json`. Set
+   `builder.json` to `{ "ios": { "path": "ios", "configuration": "Release" } }` (Debug loads the
+   JavaScript from a dev server).
+3. `builder ios build --unsigned`: the IPA lands in `./dist/`.
+4. Install [MobAI](https://mobai.run/download), connect the iPhone by USB, `builder mobai ping`,
+   then `builder mobai install dist/<file>.ipa`. It re-signs with a free Apple ID (use a spare one):
+   the app works for 7 days, then install it again.
+
+## License
+
+AGPL-3.0-or-later, as the PBX.