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/app/src/main/AndroidManifest.xml

@@ -0,0 +1,24 @@
+<?xml version="1.0" encoding="utf-8"?>
+<manifest xmlns:android="http://schemas.android.com/apk/res/android">
+
+    <uses-permission android:name="android.permission.INTERNET" />
+    <uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
+
+    <application
+        android:name=".CatacombApp"
+        android:allowBackup="false"
+        android:extractNativeLibs="true"
+        android:icon="@android:drawable/sym_def_app_icon"
+        android:label="Catacomb"
+        android:theme="@style/Theme.Catacomb">
+        <activity
+            android:name=".MainActivity"
+            android:exported="true"
+            android:theme="@style/Theme.Catacomb">
+            <intent-filter>
+                <action android:name="android.intent.action.MAIN" />
+                <category android:name="android.intent.category.LAUNCHER" />
+            </intent-filter>
+        </activity>
+    </application>
+</manifest>