feat(p2p): implement TorrServer-based P2P streaming for desktop

[aelrased]

Port the Android TorrServer-based P2P engine to Desktop (Windows and Linux).

Changes

• P2pStreamingEngine.desktop.kt: Full implementation replacing the stub
  • TorrServerBinary: manages process lifecycle (start, stop, health check)
  • TorrServerApi: HTTP REST client for TorrServer JSON API using Ktor CIO + kotlinx.serialization
  • Stream management with generation counter pattern, file index resolution, stats polling
  • Binary extraction from classpath resources at runtime (same pattern as NativePlayerBridge)
• AppFeaturePolicy.desktop.kt: Enabled p2pEnabled = true
• build.gradle.kts: Added downloadTorrserver Gradle task + JAR bundling for Linux

How it works

1. Bundles TorrServer binary (torrserver for Linux, torrserver.exe for Windows) in JAR resources at /native/torrserver/
2. Starts TorrServer as a local HTTP server on port 8091 via ProcessBuilder
3. Player streams from http://127.0.0.1:8091/stream?link=...&index=...&play

Testing

• Built with ./gradlew :composeApp:compileKotlinDesktop
• P2P toggle appears in settings
• TorrServer binary is extracted and started
• Stream URL is returned and playback attempts to start