build: add reproducible Android release verification
Resolved a conflict in android/app/build.gradle.kts. #19 was written against
the pre-#10 file and re-added the pub-cache patch tasks that #10 deleted; only
`sha256Hex()` was kept from that block, since `verifyReleaseAppConfig` needs it.
Also restored `import java.io.File`, which #10 removed along with the patch
tasks. #19's `sha256Hex(file: File)` names the type in its signature, and
Kotlin does not auto-import `java.io.File`, so the release build would have
failed at Gradle configuration without it.
Verified after resolution: no orphaned references to `projectRootDir`,
`localProperties`, `flutterSdkPath` or `patchFlutterVless`; both tests in
test/release_verification_regression_test.dart pass; `flutter analyze` reports
no errors or warnings.
Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_019HT2x2JvJNg7XNsJubfHFf
