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>

app/build.gradle.kts

@@ -2,7 +2,6 @@ import org.yaml.snakeyaml.Yaml
 
 plugins {
     id("com.android.application")
-    id("org.jetbrains.kotlin.android")
 }
 
 @Suppress("UNCHECKED_CAST")
@@ -11,18 +10,20 @@ val webviews: Map<String, Map<String, String>> =
 
 android {
     namespace = "cz.c3c.webviewkiosk"
-    compileSdk = 34
+    compileSdk = 35
+    buildToolsVersion = "36.0.0"
 
     defaultConfig {
         applicationId = "cz.c3c.webviewkiosk"
         minSdk = 26 // Sony KD-65XE9305 final firmware = Android 8.0
-        targetSdk = 34
+        targetSdk = 35
         versionCode = 1 // bump on every release; adb install -r refuses downgrades
         versionName = "0.1.0"
     }
 
     buildFeatures {
         buildConfig = true
+        resValues = true
     }
 
     signingConfigs {
@@ -54,22 +55,19 @@ android {
         }
     }
 
-    @Suppress("DEPRECATION")
-    applicationVariants.all {
-        val flavor = productFlavors.first().name
-        val type = buildType.name
-        outputs.all {
-            (this as com.android.build.gradle.internal.api.BaseVariantOutputImpl)
-                .outputFileName = "$flavor-$type.apk"
-        }
-    }
-
     compileOptions {
-        sourceCompatibility = JavaVersion.VERSION_17
-        targetCompatibility = JavaVersion.VERSION_17
+        sourceCompatibility = JavaVersion.VERSION_21
+        targetCompatibility = JavaVersion.VERSION_21
     }
-    kotlinOptions {
-        jvmTarget = "17"
+}
+
+androidComponents {
+    onVariants { variant ->
+        variant.outputs.forEach { output ->
+            val flavor = variant.productFlavors.firstOrNull()?.second ?: variant.name
+            val buildType = variant.buildType ?: "debug"
+            output.outputFileName.set("$flavor-$buildType.apk")
+        }
     }
 }