fix: 关闭悬浮底栏时LiquidGlass回退导致卡死

将LiquidGlass→Haze回退逻辑从SystemMenuPage的双Intent分发移至
ViewModel的FloatingBottomBar处理器中，确保readMenuFloatingBottomBar
和readMenuBottomBarBlurMode在同一_uiState.update中原子更新，
避免渲染管线看到floating=false+blurMode=LiquidGlass的非法中间状态。

app/src/main/java/io/legado/app/ui/book/read/ReadBookViewModel.kt

@@ -2531,10 +2531,26 @@ class ReadBookViewModel(
             }
             is ConfigUpdate.FloatingBottomBar -> {
                 ReadBookConfig.readMenuFloatingBottomBar = update.value
+                val needsBlurFallback = !update.value &&
+                        ReadBookConfig.readMenuBottomBarBlurMode == ReadMenuBlurMode.LiquidGlass
+                if (needsBlurFallback) {
+                    ReadBookConfig.readMenuBottomBarBlurMode = ReadMenuBlurMode.Haze
+                }
                 viewModelScope.launch {
                     readSettingsRepository.setReadMenuFloatingBottomBar(update.value)
+                    if (needsBlurFallback) {
+                        readSettingsRepository.setReadMenuBottomBarBlurMode(ReadMenuBlurMode.Haze)
+                    }
+                }
+                _uiState.update {
+                    it.copy(
+                        menuConfig = it.menuConfig.copy(
+                            readMenuFloatingBottomBar = update.value,
+                            readMenuBottomBarBlurMode = if (needsBlurFallback) ReadMenuBlurMode.Haze
+                            else it.menuConfig.readMenuBottomBarBlurMode,
+                        )
+                    )
                 }
-                _uiState.update { it.copy(menuConfig = it.menuConfig.copy(readMenuFloatingBottomBar = update.value)) }
             }
             is ConfigUpdate.MenuTopBarBlurMode -> {
                 val mode = update.value.coerceIn(0, 2).let {

app/src/main/java/io/legado/app/ui/book/read/sheet/SystemMenuPage.kt

@@ -525,15 +525,6 @@ private fun BottomBarTab(
             title = stringResource(R.string.read_menu_floating_bottom_bar),
             checked = floatingBottomBar,
             onCheckedChange = {
-                if (!it && preferences.readMenuBottomBarBlurMode == ReadMenuBlurMode.LiquidGlass) {
-                    onIntent(
-                        ReadBookIntent.UpdateConfig(
-                            ConfigUpdate.MenuBottomBarBlurMode(
-                                ReadMenuBlurMode.Haze
-                            )
-                        )
-                    )
-                }
                 onIntent(ReadBookIntent.UpdateConfig(ConfigUpdate.FloatingBottomBar(it)))
             },
         )