Restructure on the Android Studio project layout and update the toolchain

[in-jun]

Moves the project onto the standard Android Studio layout and the current toolchain, so that remote-sync and port-relay-android are built the same way from the same version catalog. Behaviour is unchanged except where a deprecated API had to go.

• Toolchain — Gradle 9.7.1, AGP 9.4.0, Kotlin 2.4.20, Compose BOM 2026.08.00, compileSdk/targetSdk 37. gradle/libs.versions.toml is byte-identical to the one in port-relay-android.
• Layout — java/ source sets in both modules, theme/, ui/<feature>/, per-module .gitignore, backup and data-extraction rules that exclude every app file (the config store holds SMB credentials).
• Navigation 3 — NavigationKeys + MainNavigation replace the hand-rolled Screen enum in MainActivity. The back stack is saveable, so the setup form survives process death; the first-run picker stays the root until a pair exists, and Home becomes the root once one does.
• SecurePreferences — Jetpack Security is deprecated as of 1.1.0, so pairs and settings now go through a small AES-256-GCM store keyed in the Android Keystore, with the preference key bound as GCM associated data. The old remote-sync-secure file is deleted on first load; pre-release installs re-enter their pairs.
• Battery exemption — REQUEST_IGNORE_BATTERY_OPTIMIZATIONS is dropped; Settings now opens the system optimization list instead of the direct dialog.
• R8 — the release build is minified and resource-shrunk (14.8 MB → 3.0 MB). proguard-rules.pro keeps smbj and declares the exact javax.el / org.ietf.jgss classes its dormant EL-filter and Kerberos paths reference; neither exists on Android and neither path is taken with NTLM.
• Zero-warning build — allWarningsAsErrors in both modules and lint { warningsAsErrors = true }. The one acknowledged lint is ScopedStorage on MANAGE_EXTERNAL_STORAGE; the manifest comment records why SAF cannot replace it yet (no atomic replace-rename) and that it is the planned follow-up.
• Tests — the 96 existing JVM tests pass unchanged. New: a Compose UI test for ProtocolPickerScreen and an instrumented round-trip/tamper test for SecurePreferences (compile-checked only; no device in this run).
• CI — unit tests and lint run before assembleRelease; lint and test reports are uploaded alongside the APK.

Not yet verified on a device: the R8-minified release path through an actual SMB sync.

Release signing (second commit) — app/build.gradle.kts loads keystore.properties from the project root (or ~/.android/release/ on a developer machine) exactly as the Android signing guide describes, and leaves the release unsigned when neither exists. release.yml runs on a vX.Y.Z tag: it writes keystore.properties from the four SIGNING_* secrets, runs tests and lint, builds with -PreleaseVersion=X.Y.Z (versionName from the tag, versionCode derived as major*10000 + minor*100 + patch), verifies the signature with apksigner, publishes the APK to a GitHub Release with the preinstalled gh CLI, and deletes the keystore in an always() step. Verified locally: the signed APK's certificate digest matches the keystore.