Fix NPE in update download dialog when progress is null

The download progress indicator passed a deferred lambda
`progress = { progress!! }` reading UpdateState.downloadProgress
(MutableState<Float?>). Material3 invokes the lambda during
layout/draw, outside the composition snapshot that guarded it, so it
re-read the live state. ApkDownloader sets the value to null when the
response has no content length (total <= 0), and it can also flip while
the screen is being torn down, causing the `!!` to throw NPE.

Capture the value into an immutable local before branching so the
lambda closes over a non-null Float instead of re-reading live state.

app/src/main/java/io/nekohasekai/sfa/compose/MainActivity.kt

@@ -665,16 +665,17 @@ class MainActivity :
                             )
                         } else {
                             val progress by UpdateState.downloadProgress
+                            val progressValue = progress
                             Column {
-                                if (progress != null) {
+                                if (progressValue != null) {
-                                    Text("${stringResource(R.string.downloading)} ${(progress!! * 100).toInt()}%")
+                                    Text("${stringResource(R.string.downloading)} ${(progressValue * 100).toInt()}%")
                                 } else {
                                     Text(stringResource(R.string.downloading))
                                 }
                                 Spacer(modifier = Modifier.height(8.dp))
-                                if (progress != null) {
+                                if (progressValue != null) {
                                     LinearProgressIndicator(
-                                        progress = { progress!! },
+                                        progress = { progressValue },
                                         modifier = Modifier.fillMaxWidth(),
                                     )
                                 } else {

app/src/main/java/io/nekohasekai/sfa/compose/screen/settings/AppSettingsScreen.kt

@@ -281,16 +281,17 @@ fun AppSettingsScreen(
                         )
                     } else {
                         val progress by UpdateState.downloadProgress
+                        val progressValue = progress
                         Column {
-                            if (progress != null) {
+                            if (progressValue != null) {
-                                Text("${stringResource(R.string.downloading)} ${(progress!! * 100).toInt()}%")
+                                Text("${stringResource(R.string.downloading)} ${(progressValue * 100).toInt()}%")
                             } else {
                                 Text(stringResource(R.string.downloading))
                             }
                             Spacer(modifier = Modifier.height(8.dp))
-                            if (progress != null) {
+                            if (progressValue != null) {
                                 LinearProgressIndicator(
-                                    progress = { progress!! },
+                                    progress = { progressValue },
                                     modifier = Modifier.fillMaxWidth(),
                                 )
                             } else {