Fix: back button on finder screen exits app instead of closing finder

The hot/cold FinderScreen is a full-screen overlay with no BackHandler, so system-back fell through to the Activity and finished it. Added BackHandler(onBack = onClose) so back closes the finder and returns to the main list. Release 0.1.3.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0187UiEtasyowhEBYs6s9iF5

app/build.gradle.kts

@@ -13,8 +13,8 @@ android {
         applicationId = "org.soulstone.vigil"
         minSdk = 26
         targetSdk = 35
-        versionCode = 3
-        versionName = "0.1.2"
+        versionCode = 4
+        versionName = "0.1.3"
     }
 
     // Fixed debug keystore committed to the repo (a debug key is non-secret — its