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

app/src/main/java/com/example/research/core/domain/model/IndexingProgress.kt

@@ -1,5 +1,7 @@
 package com.example.research.core.domain.model
 
+import androidx.compose.runtime.Immutable
+
 /**
  * Snapshot of an ongoing indexing operation.
  *
@@ -7,6 +9,7 @@ package com.example.research.core.domain.model
  * aggregates a file-size-weighted sum across files incrementally while
  * indexing, so reading it is O(1).
  */
+@Immutable
 data class IndexingProgress(
     val isIndexing: Boolean = false,
     val progress: Float = 0f,

app/src/main/java/com/example/research/feature/download/model/DownloadState.kt

@@ -1,4 +1,8 @@
 package com.example.research.feature.download.model
+
+import androidx.compose.runtime.Immutable
+
+@Immutable
 sealed class DownloadState {
     data object Idle : DownloadState()
     data object Loading : DownloadState()

app/src/main/java/com/example/research/ui/settings/ImportState.kt

@@ -1,5 +1,8 @@
 package com.example.research.ui.settings
 
+import androidx.compose.runtime.Immutable
+
+@Immutable
 sealed interface ImportState {
     data object Idle : ImportState
     data class Importing(val progress: Float) : ImportState

app/src/main/java/com/example/research/ui/settings/SettingsUiState.kt

@@ -1,6 +1,7 @@
 package com.example.research.ui.settings
 
 import android.net.Uri
+import androidx.compose.runtime.Immutable
 import com.example.research.core.domain.model.AppTheme
 import com.example.research.core.domain.model.Dictionary
 import com.example.research.core.domain.model.DictionarySource
@@ -8,6 +9,7 @@ import com.example.research.core.domain.model.IndexingProgress
 import com.example.research.DictionaryStatus
 import com.example.research.feature.download.model.DownloadState
 
+@Immutable
 data class SettingsUiState(
     val theme: AppTheme = AppTheme.SYSTEM,
     val language: String = "system",