ci(android): fix CI runner (JDK/SDK/gradlew) + variant test

PR #2 merged the M0 scaffold with the initial CI, which fails on this
self-hosted runner. Fix forward:

- JDK 17 via apt (openjdk-17-jdk-headless) + JAVA_HOME, replacing
  actions/setup-java: the runner can't resolve api.adoptium.net (EAI_AGAIN).
- Explicit `sdkmanager` install of platform-tools + platforms;android-35 +
  build-tools;35.0.0 so the build never relies on AGP auto-download.
- `chmod +x ./gradlew` in the run step (and set the 100755 git bit): this
  runner's checkout drops file modes, so `./gradlew` alone hit
  "Permission denied" (exit 126).
- ScaffoldSanityTest: drop the BuildConfig.DEBUG assertion, which fails under
  testReleaseUnitTest (the `test` task runs both variants); assert VERSION_NAME.
- Remove the redundant android:label on MainActivity (RedundantLabel lint).

Verified locally against the Android Studio SDK (platform 35 + build-tools
35.0.0): `./gradlew lint test assembleDebug` passes and builds a debug APK.

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

app/src/test/java/com/runicgateway/app/ScaffoldSanityTest.kt

@@ -10,6 +10,9 @@ import org.junit.Test
 /**
  * Placeholder JVM unit test so the `test` CI gate has something to run in M0.
  * Real repository / view-model tests arrive with the functional pass (M1+).
+ *
+ * Assertions must be variant-agnostic: the `test` task runs both the debug and
+ * release unit-test variants, so nothing here may depend on `BuildConfig.DEBUG`.
  */
 class ScaffoldSanityTest {
     @Test
@@ -18,8 +21,7 @@ class ScaffoldSanityTest {
     }
 
     @Test
-    fun buildConfigIsDebuggableInTest() {
-        // Unit tests run against the debug variant.
-        assertTrue(BuildConfig.DEBUG)
+    fun versionNameIsSet() {
+        assertTrue(BuildConfig.VERSION_NAME.isNotBlank())
     }
 }