feat(android): Compose app with bundled yt-dlp, theme pack, nav & progress

Turn android/ into a real Gradle app (Kotlin + Jetpack Compose + Material3)
that bundles an on-device yt-dlp engine and reuses the Phase-4 Rust JNI core.

- Bundled engine: youtubedl-android (Python + yt-dlp + ffmpeg + aria2c) via
  Engine.kt; initialises off the main thread on first launch. Requires legacy
  jniLib packaging so the bundled .zip.so payloads extract to disk.
- Intuitive navigation: bottom NavigationBar — Download / Files / Settings.
- Settings section: dedicated screen with a live theme picker, default-quality
  chips, engine info, and a Rust-core demo.
- All 19 desktop themes ported from src/theme.rs into Theme.kt as Material3
  colour schemes, shown as tappable swatches; selection persisted (Prefs.kt).
- Download screen: live platform detection through the Rust core as you type,
  plus an improved animated determinate progress indicator (%/ETA/status/
  success-error) and a scrollable log.

Toolchain pinned for reproducibility: Gradle 8.9 wrapper, AGP 8.5.2, Kotlin
1.9.24, Compose BOM 2024.06.00, compileSdk 34, minSdk 24, ABIs arm64-v8a +
x86_64. build-apk.sh runs Gradle on a JDK <= 21 (system JDK may break R8) and
builds the Rust libs first.

Verified on an Android 14 x86_64 emulator: installs, launches, engine reports
"yt-dlp ready", Rust .so loads, theme switching re-themes live and persists
across restart, all screens render. (A real YouTube download still depends on
the anti-bot/POT question — Phases 2-3, device-only.)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

android/gradle.properties

@@ -0,0 +1,7 @@
+# Portable Gradle settings. The JDK is supplied via JAVA_HOME at invocation
+# time (build-apk.sh pins JDK 17) rather than hardcoding a machine path here.
+org.gradle.jvmargs=-Xmx2048m -Dfile.encoding=UTF-8
+org.gradle.caching=true
+android.useAndroidX=true
+android.nonTransitiveRClass=true
+kotlin.code.style=official