Pin Android SDK components and aapt2 override for Flutter builds

The Android SDK lives in the read-only Nix store, so the Gradle build
cannot auto-install missing components. Pin everything the Flutter
Android build needs in the dev shell's composeAndroidPackages:
platforms 33-36 (app targets 36; plugins pin 34 and 35), build-tools
35.0.0 (AGP 8.11.1), NDK 28.2.13676358 (flutter.ndkVersion), and
cmake 3.22.1 (plugin native builds).

Also redirect the Android Gradle Plugin to the Nix-patched aapt2 from
the SDK via android.aapt2FromMavenOverride, written to the user-global
~/.gradle/gradle.properties on shell entry (the project's tracked
gradle.properties must not contain a machine-specific store path).
AGP's Maven-downloaded aapt2 cannot run on NixOS.

Ignore the generated frontend/android/build/ directory.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

flake.nix

@@ -39,11 +39,17 @@
       # available and reproducible across machines, instead of relying on the
       # contents of the default androidPkgs bundle (which can drift over time).
       androidComposition = pkgsBySystem.${system}.androidenv.composeAndroidPackages {
-        platformVersions = ["36"]; # matches Flutter 3.41 compileSdk/targetSdk
+        # 36 = Flutter 3.41 app compileSdk/targetSdk; 33-35 cover plugin subprojects
+        # which each pin their own compileSdk (e.g. package_info_plus -> 34, jni -> 35).
+        platformVersions = ["33" "34" "35" "36"];
+        buildToolsVersions = ["35.0.0"]; # required by Android Gradle Plugin 8.11.1
         includeEmulator = true;
         includeSystemImages = true;
         systemImageTypes = ["google_apis"];
         abiVersions = ["x86_64"];
+        includeNDK = true;
+        ndkVersions = ["28.2.13676358"]; # matches Flutter 3.41 flutter.ndkVersion
+        cmakeVersions = ["3.22.1"]; # required by plugin native (externalNativeBuild) tasks
       };
     in {
       default = pkgsBySystem.${system}.mkShell rec {
@@ -87,6 +93,22 @@
               || echo "  WARNING: failed to create Android emulator '$ANDROID_AVD_NAME'."
           fi
 
+          # Point the Android Gradle Plugin at the Nix-patched aapt2 shipped in
+          # the SDK build-tools. AGP otherwise downloads a prebuilt aapt2 from
+          # Maven that cannot run on NixOS (its ELF interpreter is missing).
+          # We write this to the user-global Gradle properties because the
+          # project's android/gradle.properties is version-controlled and must
+          # not contain a machine-specific Nix store path. The managed block is
+          # rewritten on every entry so it tracks the pinned SDK store path.
+          aapt2Bin="$(ls "$ANDROID_SDK_ROOT"/build-tools/*/aapt2 2>/dev/null | sort | tail -n1)"
+          if [ -n "$aapt2Bin" ]; then
+            mkdir -p "$HOME/.gradle"
+            gradleProps="$HOME/.gradle/gradle.properties"
+            touch "$gradleProps"
+            sed -i '/# >>> oott-nix >>>/,/# <<< oott-nix <<</d' "$gradleProps"
+            printf '# >>> oott-nix >>>\nandroid.aapt2FromMavenOverride=%s\n# <<< oott-nix <<<\n' "$aapt2Bin" >> "$gradleProps"
+          fi
+
           DEV_SHELL=oott exec fish
         '';
       };

frontend/android/.gitignore

@@ -1,5 +1,6 @@
 gradle-wrapper.jar
 /.gradle
+/build/
 /captures/
 /gradlew
 /gradlew.bat