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>

README.md

@@ -25,10 +25,10 @@ Each entry produces a separate APK with a unique `applicationId`
 ## Build
 
     # all flavors
-    nix develop --command gradle --no-daemon :app:assembleRelease
+    gradle --no-daemon :app:assembleRelease
 
     # single flavor
-    nix develop --command gradle --no-daemon :app:assembleWeatherRelease
+    gradle --no-daemon :app:assembleWeatherRelease
 
 APKs: `app/build/outputs/apk/<flavor>/release/<flavor>-release.apk`