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/build.gradle.kts

@@ -0,0 +1,100 @@
+import java.util.Properties
+
+plugins {
+    id("com.android.application")
+    id("org.jetbrains.kotlin.android")
+}
+
+android {
+    namespace = "com.catacomb.spike"
+    compileSdk = 34
+
+    defaultConfig {
+        applicationId = "com.catacomb.spike"
+        minSdk = 24
+        targetSdk = 34
+        versionCode = 2
+        versionName = "0.2-spike"
+        // youtubedl-android ships native Python for these ABIs; the Rust core
+        // (jniLibs/) ships arm64-v8a + x86_64. Restrict to the intersection so
+        // every ABI in the APK has both libraries.
+        ndk {
+            abiFilters += listOf("arm64-v8a", "x86_64")
+        }
+    }
+
+    signingConfigs {
+        // Reproducible debug key so `assembleDebug` produces an installable,
+        // stably-signed APK without Android Studio. Generated on first build
+        // by build-apk.sh if absent.
+        create("debugks") {
+            val ks = rootProject.file("debug.keystore")
+            if (ks.exists()) {
+                storeFile = ks
+                storePassword = "android"
+                keyAlias = "androiddebugkey"
+                keyPassword = "android"
+            }
+        }
+    }
+
+    buildTypes {
+        getByName("debug") {
+            isMinifyEnabled = false
+            if (rootProject.file("debug.keystore").exists()) {
+                signingConfig = signingConfigs.getByName("debugks")
+            }
+        }
+        getByName("release") {
+            isMinifyEnabled = false
+            signingConfig = signingConfigs.getByName("debug")
+        }
+    }
+
+    compileOptions {
+        sourceCompatibility = JavaVersion.VERSION_17
+        targetCompatibility = JavaVersion.VERSION_17
+    }
+    kotlinOptions {
+        jvmTarget = "17"
+    }
+    buildFeatures {
+        compose = true
+    }
+    composeOptions {
+        // Matches Kotlin 1.9.24.
+        kotlinCompilerExtensionVersion = "1.5.14"
+    }
+    packaging {
+        resources {
+            excludes += "/META-INF/{AL2.0,LGPL2.1}"
+        }
+        // youtubedl-android extracts its bundled Python/ffmpeg/aria2c payloads
+        // (libpython.zip.so, …) from the native lib dir at first launch, so the
+        // .so files MUST be extracted to disk — i.e. legacy packaging on
+        // (extractNativeLibs=true). With them left compressed-in-APK the engine
+        // init fails.
+        jniLibs {
+            useLegacyPackaging = true
+        }
+    }
+}
+
+dependencies {
+    val composeBom = platform("androidx.compose:compose-bom:2024.06.00")
+    implementation(composeBom)
+    implementation("androidx.core:core-ktx:1.13.1")
+    implementation("androidx.activity:activity-compose:1.9.0")
+    implementation("androidx.lifecycle:lifecycle-runtime-ktx:2.8.2")
+    implementation("androidx.lifecycle:lifecycle-viewmodel-compose:2.8.2")
+    implementation("androidx.compose.ui:ui")
+    implementation("androidx.compose.ui:ui-graphics")
+    implementation("androidx.compose.material3:material3")
+    implementation("androidx.compose.material:material-icons-extended")
+
+    // On-device yt-dlp engine (bundled Python + yt-dlp) + ffmpeg + aria2c.
+    implementation("io.github.junkfood02.youtubedl-android:library:0.18.1")
+    implementation("io.github.junkfood02.youtubedl-android:ffmpeg:0.18.1")
+    implementation("io.github.junkfood02.youtubedl-android:aria2c:0.18.1")
+    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-android:1.8.1")
+}