fix(core): read-only editor does not support code preview (#12700)

Close [AI-160](https://linear.app/affine-design/issue/AI-160)

<!-- This is an auto-generated comment: release notes by coderabbit.ai -->

## Summary by CodeRabbit

- **New Features**
  - Improved preview state management for code blocks, ensuring consistent behavior in both editable and readonly modes.

- **Refactor**
  - Streamlined the way preview state is toggled and displayed for code blocks, resulting in a more reliable and maintainable user experience.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

blocksuite/affine/blocks/code/src/code-block.ts

@@ -40,6 +40,16 @@ export class CodeBlockComponent extends CaptionedBlockComponent<CodeBlockModel>
 
   private _inlineRangeProvider: InlineRangeProvider | null = null;
 
+  private readonly _localPreview$ = signal<boolean | null>(null);
+
+  preview$: Signal<boolean> = computed(() => {
+    const modelPreview = !!this.model.props.preview$.value;
+    if (this.store.readonly) {
+      return this._localPreview$.value ?? modelPreview;
+    }
+    return modelPreview;
+  });
+
   highlightTokens$: Signal<ThemedToken[][]> = signal([]);
 
   languageName$: Signal<string> = computed(() => {
@@ -393,7 +403,7 @@ export class CodeBlockComponent extends CaptionedBlockComponent<CodeBlockModel>
         true) &&
       (this.model.props.lineNumber ?? true);
 
-    const preview = !!this.model.props.preview;
+    const preview = this.preview$.value;
     const previewContext = this.std.getOptional(
       CodeBlockPreviewIdentifier(this.model.props.language ?? '')
     );
@@ -461,6 +471,14 @@ export class CodeBlockComponent extends CaptionedBlockComponent<CodeBlockModel>
   override accessor useCaptionEditor = true;
 
   override accessor useZeroWidth = true;
+
+  setPreviewState(preview: boolean) {
+    if (this.store.readonly) {
+      this._localPreview$.value = preview;
+    } else {
+      this.store.updateBlock(this.model, { preview });
+    }
+  }
 }
 
 declare global {

blocksuite/affine/blocks/code/src/code-toolbar/components/preview-button.ts

@@ -58,11 +58,7 @@ export class PreviewButton extends WithDisposable(SignalWatcher(LitElement)) {
   `;
 
   private readonly _toggle = (value: boolean) => {
-    if (this.blockComponent.store.readonly) return;
+    this.blockComponent.setPreviewState(value);
-
-    this.blockComponent.store.updateBlock(this.blockComponent.model, {
-      preview: value,
-    });
 
     const std = this.blockComponent.std;
     const mode = std.getOptional(DocModeProvider)?.getEditorMode() ?? 'page';
@@ -77,7 +73,7 @@ export class PreviewButton extends WithDisposable(SignalWatcher(LitElement)) {
   };
 
   get preview() {
-    return !!this.blockComponent.model.props.preview$.value;
+    return this.blockComponent.preview$.value;
   }
 
   override render() {