Quality & Performance improvements: error handling, speed optimization, peer discovery, battery optimization

[jpcottin]

Summary

Comprehensive Quality & Performance improvements addressing four key pillars:

1. Better Error Handling — Network timeouts, tracker/torrent error alerts, per-torrent error display, snackbar feedback for addMagnet failures
2. Download Speed Optimization — Connection limits, unchoke slots, active torrent limits, send buffer tuning
3. Improved Peer Discovery — DHT bootstrap nodes configured, announce_to_all_trackers/tiers enabled
4. Battery & Memory Optimization — Adaptive tick interval (500ms foreground / 2000ms background), polling rate adjustment, alert queue capping

Technical Details

JNI (torrent_jni.cpp)

• Added timeouts: peer 60s, request 20s, handshake 10s
• Connection limits: 200 total, 8 unchoke slots, 4 active downloads, 4 active seeds
• Send buffer tuning: 512KB watermark, 128KB low watermark
• DHT bootstrap nodes: router.bittorrent.com, router.utorrent.com, dht.transmissionbt.com
• Alert processing: collect torrent/tracker errors into per-torrent lastError field
• Tick interval: 500ms (configurable via nativeSetTickInterval)

Kotlin Layer

• TorrentManager: added lastError field, isInBackground flag, JNI declarations
• DataRepository: retryWhen with exponential backoff, flowOn(IO), adaptive polling delay
• MainActivity: lifecycle-aware tick interval control (onStart/onStop)
• MainScreenViewModel: errorEvents SharedFlow for snackbar feedback

UI

• MainScreen: error display on torrent cards (red text, truncated), snackbar alerts
• All error paths now surface to user instead of silent failures

Testing & Docs

• Added error handling test to MainScreenViewModelTest
• All 26 unit tests pass, lint clean
• README updated with Recent Improvements section

Verification

• ✅ ./gradlew testDebugUnitTest — all 26 tests pass
• ✅ ./gradlew lint — no warnings
• ✅ App installs and launches on R37 and MP36 emulators
• ✅ Add valid torrent — downloads normally
• ✅ Add invalid magnet — snackbar error appears
• ✅ Check torrent with bad tracker — lastError text on card
• ✅ Background app → verify tick_interval slows to 2000ms
• ✅ Foreground app → verify tick_interval restores to 500ms