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/build-apk.sh

@@ -0,0 +1,59 @@
+#!/usr/bin/env bash
+# Build the Catacomb Android app (Compose + bundled yt-dlp + Rust JNI core).
+#
+# Runs Gradle on a JDK it tolerates (<= 21; the system JDK may be newer and
+# breaks R8/d8) and ensures the Rust native libs exist first. Produces a
+# debug-signed, installable APK.
+#
+# Usage: ./build-apk.sh
+# Output: app/build/outputs/apk/debug/app-debug.apk
+set -euo pipefail
+cd "$(dirname "$0")"
+
+# ── JDK for Gradle/AGP (<= 21) ────────────────────────────────────────────
+find_jdk() {
+    if [[ -n "${TOOL_JAVA_HOME:-}" && -x "$TOOL_JAVA_HOME/bin/java" ]]; then
+        echo "$TOOL_JAVA_HOME"; return
+    fi
+    local c
+    for c in java-17-openjdk java-21-openjdk; do
+        [[ -x "/usr/lib/jvm/$c/bin/java" ]] && { echo "/usr/lib/jvm/$c"; return; }
+    done
+    echo "${JAVA_HOME:-}"
+}
+export JAVA_HOME="$(find_jdk)"
+[[ -x "$JAVA_HOME/bin/java" ]] || { echo "ERROR: need a JDK <= 21 (set TOOL_JAVA_HOME)."; exit 1; }
+echo "Using JDK: $JAVA_HOME"
+
+# ── Android SDK location for Gradle ───────────────────────────────────────
+SDK="${ANDROID_HOME:-${ANDROID_SDK_ROOT:-$HOME/Android/Sdk}}"
+[[ -d "$SDK" ]] || { echo "ERROR: Android SDK not found (set ANDROID_HOME)."; exit 1; }
+if [[ ! -f local.properties ]]; then
+    echo "sdk.dir=$SDK" > local.properties
+fi
+
+# ── Native libs (Rust core) ───────────────────────────────────────────────
+if ! ls app/src/main/jniLibs/*/libcatacomb_core.so >/dev/null 2>&1; then
+    echo "Building Rust core native libs…"
+    ( cd rust/catacomb_core && ./build.sh )
+fi
+
+# ── Debug keystore (stable, reproducible signing) ─────────────────────────
+if [[ ! -f debug.keystore ]]; then
+    "$JAVA_HOME/bin/keytool" -genkeypair -keystore debug.keystore \
+        -alias androiddebugkey -storepass android -keypass android \
+        -keyalg RSA -keysize 2048 -validity 10000 \
+        -dname "CN=Android Debug,O=Android,C=US" >/dev/null 2>&1
+fi
+
+echo "── gradlew assembleDebug ─────────────────────────────────────"
+./gradlew --no-daemon assembleDebug "$@"
+
+APK="app/build/outputs/apk/debug/app-debug.apk"
+if [[ -f "$APK" ]]; then
+    echo ""
+    echo "✓ APK: $(pwd)/$APK  ($(du -h "$APK" | cut -f1))"
+    echo "  Install: adb install -r $APK"
+else
+    echo "✗ build finished but $APK not found" >&2; exit 1
+fi