feat: add quadrator mode — 4 simultaneous camera streams in 2x2 grid

- QuadCell inner class: self-contained RTSP video-only player per cell
   - TCP-only in quad mode (UDP fixed ports can't be shared)
   - No audio in quad mode to reduce resource usage
   - Per-camera Quad: YES/NO toggle in Settings (default NO)
   - Quadrator: OFF/ON menu item between Carousel and Exit
   - Quad and Carousel are mutually exclusive
   - State persisted in SharedPreferences, survives pause/resume
   - Tap any quadrant to open menu
   - Bump version to 1.17

Author: ystinia

app/build.gradle

@@ -18,8 +18,8 @@ android {
         applicationId "com.openipc.decoder"
         minSdk 21
         targetSdk 36
-        versionCode 16
-        versionName "1.16"
+        versionCode 17
+        versionName "1.17"
     }
 
     // Signing credentials from environment variables; null when absent.