feat: eye-catching update banner + cross-channel notify + periodic check

- HomeScreen update banner: gradient background (tertiary→primary), white
  text, large version number, rocket emoji CTA button
- UpdateChecker now auto-notifies via all enabled channels (Bark/ntfy/
  email) when a new version is discovered, with version/size/notes
- Periodic check every 24h via coroutine delay loop (replaces one-shot
  startup check only); also honors updateCheckEnabled toggle
- version 1.7.0 -> 1.7.1 (code 13 -> 14)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/build.gradle.kts

@@ -13,8 +13,8 @@ android {
         applicationId = "com.a2i.forwarder"
         minSdk = 34
         targetSdk = 36
-        versionCode = 13
-        versionName = "1.7.0"
+        versionCode = 14
+        versionName = "1.7.1"
     }
 
     signingConfigs {