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.lock

@@ -20,16 +20,16 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1781214844,
-        "narHash": "sha256-GiKi1nonuwTHG1mTrwFTllfMSN2rvHQZUgpW0nQX/qM=",
+        "lastModified": 1781284521,
+        "narHash": "sha256-AFxK4Q2YbBXehBxJEL0IWTS+pauHY1w6O/GBUijTRF0=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "9bc9b4b4e7b1e7ce7ec2a8355b4369541b90cd6a",
+        "rev": "a06029d13f9b4a70db3aae42d4f233e6f6c92075",
         "type": "github"
       },
       "original": {
         "owner": "NixOS",
-        "ref": "release-25.11",
+        "ref": "release-26.05",
         "repo": "nixpkgs",
         "type": "github"
       }