chore: upgrade toolchain to AGP 9.2.1 / Gradle 9 / nixpkgs 26.05

- nixpkgs 25.11 → 26.05; gradle_9 + jdk21; build-tools 35 → 36
- Drop custom gradle wrapper — GRADLE_OPTS works post-nixpkgs PR #449037
- AGP 8.7.3 → 9.2.1; Kotlin plugin removed (built-in since AGP 9)
- compileSdk/targetSdk 34 → 35; buildToolsVersion pinned to 36.0.0
- Java/Kotlin target 17 → 21; resValues feature flag required by AGP 9
- applicationVariants (removed in AGP 9) → androidComponents.onVariants
- Test task: testWeatherReleaseUnitTest → :app:test (all flavors, debug)
- CI artifact path fixed to cover all flavor APKs
- Docs: drop nix develop --command prefix (direnv activates shell)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

flake.nix

@@ -9,7 +9,7 @@
   description = "Android TV fullscreen WebView kiosk for the Grafana house dashboard";
 
   inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs/release-25.11";
+    nixpkgs.url = "github:NixOS/nixpkgs/release-26.05";
     flake-utils.url = "github:numtide/flake-utils";
   };
 
@@ -30,10 +30,10 @@
           };
         };
 
-        buildToolsVersion = "34.0.0";
+        buildToolsVersion = "36.0.0";
 
         androidComposition = pkgs.androidenv.composeAndroidPackages {
-          platformVersions = [ "34" ];
+          platformVersions = [ "35" ];
           buildToolsVersions = [ buildToolsVersion ];
           includeEmulator = false;
           includeSystemImages = false;
@@ -43,36 +43,25 @@
         sdkRoot = "${androidSdk}/libexec/android-sdk";
 
         fonts = with pkgs; [ noto-fonts dejavu_fonts freefont_ttf ];
-
-        # NixOS gotcha: AGP downloads a dynamically-linked aapt2 from Maven
-        # that can't run on NixOS. android.aapt2FromMavenOverride is a Gradle
-        # *project* property; `-Dorg.gradle.project.<name>` on the command line
-        # sets one. It must be a CLI arg, not GRADLE_OPTS: the gradle package
-        # in nixpkgs 25.11 is a raw `exec java ... GradleMain "$@"` wrapper
-        # that ignores GRADLE_OPTS (fixed post-25.11 in nixpkgs PR #449037 —
-        # once we're past 25.11, plain GRADLE_OPTS works and this wrapper can go).
-        gradle = pkgs.symlinkJoin {
-          name = "gradle-nixos-aapt2";
-          paths = [ pkgs.gradle ];
-          nativeBuildInputs = [ pkgs.makeWrapper ];
-          postBuild = ''
-            wrapProgram $out/bin/gradle \
-              --add-flags "-Dorg.gradle.project.android.aapt2FromMavenOverride=${sdkRoot}/build-tools/${buildToolsVersion}/aapt2"
-          '';
-        };
       in
       {
         devShells.default = pkgs.mkShell {
           packages = [
             androidSdk
-            pkgs.jdk17
-            gradle # wrapped, see above
+            pkgs.jdk21
+            pkgs.gradle_9
             pkgs.imagemagick # banner/icon generation
             pkgs.android-tools # adb for sideloading
           ] ++ fonts;
 
           ANDROID_HOME = sdkRoot;
           ANDROID_SDK_ROOT = sdkRoot;
+          JAVA_HOME = "${pkgs.jdk21}";
+          # NixOS gotcha: AGP downloads a dynamically-linked aapt2 from Maven
+          # that can't run on NixOS. Set android.aapt2FromMavenOverride via
+          # GRADLE_OPTS so AGP uses the SDK's aapt2 instead. Works since
+          # nixpkgs PR #449037 (merged post-25.11, present in 26.05+).
+          GRADLE_OPTS = "-Dorg.gradle.project.android.aapt2FromMavenOverride=${sdkRoot}/build-tools/${buildToolsVersion}/aapt2";
           FONTCONFIG_FILE = pkgs.makeFontsConf { fontDirectories = fonts; };
         };
       }