Fix polling interval settings and add release build signing

- Fix polling interval not reflecting saved value (use SharingStarted.Eagerly)
- Add release build signing configuration with keystore
- Add key.properties to .gitignore for security
- Add ProGuard rules for JSpecify annotations

app/proguard-rules.pro

@@ -16,3 +16,7 @@
 # Kotlin Coroutines
 -keepnames class kotlinx.coroutines.internal.MainDispatcherFactory {}
 -keepnames class kotlinx.coroutines.CoroutineExceptionHandler {}
+
+# JSpecify annotations (for Jsoup compatibility)
+-dontwarn org.jspecify.annotations.**
+