fix: sample catalog, modern loading, signing, and dependency cleanup

[halilozel1903]

Summary

Closes remaining post-polish gaps for the Android TV leanback demo. Rebased onto master (includes merged README/CI polish from #1).

• Sample content: Browse/detail data comes from app/src/main/assets/catalog.json with bundled posters under assets/posters/ and documented Google Android TV sample fallbackUrl values. See docs/SAMPLE_CONTENT.md.
• AsyncTask: VideoDetailsFragment loads poster bitmaps on a background executor, applies UI on the main thread, and cancels work in onStop/onDestroy.
• Dependencies: Leanback bumped to 1.1.0-rc02 (compatible with AGP 8.7 / compileSdk 34). AppCompat, Fragment; Kotlin stdlib duplicate excludes for classpath sync.
• Release signing: Optional keystore.properties or ANDROID_* env vars; debug builds unchanged. Template: keystore.properties.example.
• Git hygiene: Full .idea/ ignored and removed from the index; keeps master’s *.apk / *.aab / *.hprof ignores.

Test plan

• ./gradlew assembleDebug

Risks

• Leanback 1.2.0 requires AGP 8.6+ and compileSdk 35; not applied in this PR.
• Bundled posters are placeholders (copied from movie.png); replace with real art in assets/posters/ as needed.
• Release signing is opt-in; CI release builds stay unsigned unless secrets are configured.

  

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