fix(geohash): cleanup sampling subscriptions when closing location sheet (#659)

- Added DisposableEffect to LocationChannelsSheet to ensure cleanup runs when composable is removed
- Fixed ChatViewModel.endGeohashSampling() to correctly delegate to GeohashViewModel instead of being a no-op
- Prevents lingering kind 20001 subscriptions after closing the sheet

app/src/main/java/com/bitchat/android/ui/LocationChannelsSheet.kt

@@ -542,6 +542,13 @@ fun LocationChannelsSheet(
             viewModel.endGeohashSampling()
         }
     }
+
+    // Ensure cleanup when the composable is destroyed (e.g. removed from parent composition)
+    DisposableEffect(Unit) {
+        onDispose {
+            viewModel.endGeohashSampling()
+        }
+    }
 }
 
 @Composable