perf: mark Compose state types as @Immutable

Compose compiler reports showed DownloadState's sealed root as Uncertain
and DownloadProgressState/SettingsUiState as runtime stability, so
skippability checks were deferred to runtime. The annotations make them
compile-time stable and turn any future mutable field into an explicit
contract break.

app/src/main/java/com/example/research/DictionaryStatus.kt

@@ -1,5 +1,8 @@
 package com.example.research
 
+import androidx.compose.runtime.Immutable
+
+@Immutable
 sealed interface DictionaryStatus {
     data object Unknown : DictionaryStatus
     data object Checking : DictionaryStatus