Keep the camera FAB stable across folder navigation

- Skip rendering the FAB until saved preferences load, so it is never
  drawn at the default corner and never jumps to its dragged position
- Share user preferences as one hot process-wide StateFlow instead of a
  cold disk read per screen, so a newly created gallery screen receives
  the FAB position synchronously in the ViewModel constructor

app/src/main/java/com/example/onecamera/data/preferences/UserPreferencesRepository.kt

@@ -9,8 +9,13 @@ import androidx.datastore.preferences.core.floatPreferencesKey
 import androidx.datastore.preferences.core.stringPreferencesKey
 import androidx.datastore.preferences.preferencesDataStore
 import dagger.hilt.android.qualifiers.ApplicationContext
-import kotlinx.coroutines.flow.Flow
+import kotlinx.coroutines.CoroutineScope
+import kotlinx.coroutines.Dispatchers
+import kotlinx.coroutines.SupervisorJob
+import kotlinx.coroutines.flow.SharingStarted
+import kotlinx.coroutines.flow.StateFlow
 import kotlinx.coroutines.flow.map
+import kotlinx.coroutines.flow.stateIn
 import javax.inject.Inject
 import javax.inject.Singleton
 
@@ -41,17 +46,26 @@ class UserPreferencesRepository @Inject constructor(
         val SCREEN_TRANSITIONS_ENABLED = booleanPreferencesKey("screen_transitions_enabled")
     }
 
-    val userPreferencesFlow: Flow<UserPreferences> = context.dataStore.data.map { prefs ->
-        UserPreferences(
-            rootFolderUri = prefs[Keys.ROOT_FOLDER_URI],
-            cameraFabXFraction = prefs[Keys.CAMERA_FAB_X_FRACTION]?.coerceIn(0f, 1f) ?: 1f,
-            cameraFabYFraction = prefs[Keys.CAMERA_FAB_Y_FRACTION]?.coerceIn(0f, 1f) ?: 1f,
-            gallerySortAscending = prefs[Keys.GALLERY_SORT_ASCENDING] ?: true,
-            galleryGridMode = prefs[Keys.GALLERY_GRID_MODE] ?: true,
-            zipJpegCompression = prefs[Keys.ZIP_JPEG_COMPRESSION] ?: true,
-            screenTransitionsEnabled = prefs[Keys.SCREEN_TRANSITIONS_ENABLED] ?: true,
+    // Hot, process-wide cache: the DataStore file is read once, and every screen
+    // created later receives the current value synchronously on subscription,
+    // instead of re-reading disk per screen and flashing default positions.
+    val userPreferencesFlow: StateFlow<UserPreferences> = context.dataStore.data
+        .map { prefs ->
+            UserPreferences(
+                rootFolderUri = prefs[Keys.ROOT_FOLDER_URI],
+                cameraFabXFraction = prefs[Keys.CAMERA_FAB_X_FRACTION]?.coerceIn(0f, 1f) ?: 1f,
+                cameraFabYFraction = prefs[Keys.CAMERA_FAB_Y_FRACTION]?.coerceIn(0f, 1f) ?: 1f,
+                gallerySortAscending = prefs[Keys.GALLERY_SORT_ASCENDING] ?: true,
+                galleryGridMode = prefs[Keys.GALLERY_GRID_MODE] ?: true,
+                zipJpegCompression = prefs[Keys.ZIP_JPEG_COMPRESSION] ?: true,
+                screenTransitionsEnabled = prefs[Keys.SCREEN_TRANSITIONS_ENABLED] ?: true,
+            )
+        }
+        .stateIn(
+            scope = CoroutineScope(SupervisorJob() + Dispatchers.IO),
+            started = SharingStarted.Eagerly,
+            initialValue = UserPreferences(),
         )
-    }
 
     suspend fun setRootFolderUri(uri: String) {
         context.dataStore.edit { prefs ->

app/src/main/java/com/example/onecamera/ui/gallery/GalleryScreen.kt

@@ -474,12 +474,14 @@ fun GalleryScreen(
         enter = fadeIn(tween(250)),
         exit = fadeOut(tween(150)),
     ) {
-        DraggableCameraFab(
-            xFraction = uiState.cameraFabXFraction,
-            yFraction = uiState.cameraFabYFraction,
-            onPositionChange = viewModel::saveCameraFabPosition,
-            onClick = { onNavigateToCamera(uiState.currentFolderUri ?: folderUri) },
-        )
+        if (uiState.viewPreferencesLoaded) {
+            DraggableCameraFab(
+                xFraction = uiState.cameraFabXFraction,
+                yFraction = uiState.cameraFabYFraction,
+                onPositionChange = viewModel::saveCameraFabPosition,
+                onClick = { onNavigateToCamera(uiState.currentFolderUri ?: folderUri) },
+            )
+        }
     }
 
     SelectionActionsBar(

app/src/main/java/com/example/onecamera/ui/gallery/GalleryViewModel.kt

@@ -205,6 +205,9 @@ class GalleryViewModel @Inject constructor(
         loadJob = viewModelScope.launch {
             uiState.value = uiState.value.copy(
                 currentFolderUri = folderUriString,
+                folderName = state.folderName.ifEmpty {
+                    folderUriString.substringAfterLast(':').substringAfterLast('/')
+                },
                 isLoading = true,
                 error = null,
             )