feat(web): add theme-aware Mermaid diagram rendering

Update MermaidBlock component to automatically adapt to light/dark theme
changes, ensuring diagrams remain visually consistent with the app theme.

Changes:
- Add MutationObserver to watch for data-theme attribute changes
- Map app themes to Mermaid themes (default-dark → dark, others → default)
- Re-render diagrams automatically when theme switches
- Maintain proper TypeScript typing for Mermaid theme values

The component now responds instantly to theme changes, providing a seamless
user experience when switching between light and dark modes or using
"Sync with system" option.

web/src/components/MemoContent/MermaidBlock.tsx

@@ -7,23 +7,60 @@ interface MermaidBlockProps {
   className?: string;
 }
 
-// Initialize mermaid with default configuration
-mermaid.initialize({
-  startOnLoad: false,
-  theme: "default",
-  securityLevel: "strict",
-  fontFamily: "inherit",
-});
+/**
+ * Maps app theme to Mermaid theme
+ * @param appTheme - The app's theme value from data-theme attribute
+ * @returns Mermaid theme name
+ */
+const getMermaidTheme = (appTheme: string | null): "default" | "dark" => {
+  switch (appTheme) {
+    case "default-dark":
+      return "dark";
+    case "default":
+    case "paper":
+    case "whitewall":
+    default:
+      return "default";
+  }
+};
+
+/**
+ * Gets the current theme from the document
+ */
+const getCurrentTheme = (): string => {
+  return document.documentElement.getAttribute("data-theme") || "default";
+};
 
 export const MermaidBlock = ({ children, className }: MermaidBlockProps) => {
   const containerRef = useRef<HTMLDivElement>(null);
   const [svg, setSvg] = useState<string>("");
   const [error, setError] = useState<string>("");
+  const [currentTheme, setCurrentTheme] = useState<string>(getCurrentTheme());
 
   // Extract the code element and its content
   const codeElement = children as React.ReactElement;
   const codeContent = String(codeElement?.props?.children || "").replace(/\n$/, "");
 
+  // Watch for theme changes
+  useEffect(() => {
+    const observer = new MutationObserver((mutations) => {
+      mutations.forEach((mutation) => {
+        if (mutation.type === "attributes" && mutation.attributeName === "data-theme") {
+          const newTheme = getCurrentTheme();
+          setCurrentTheme(newTheme);
+        }
+      });
+    });
+
+    observer.observe(document.documentElement, {
+      attributes: true,
+      attributeFilter: ["data-theme"],
+    });
+
+    return () => observer.disconnect();
+  }, []);
+
+  // Render diagram when content or theme changes
   useEffect(() => {
     const renderDiagram = async () => {
       if (!codeContent || !containerRef.current) {
@@ -34,6 +71,17 @@ export const MermaidBlock = ({ children, className }: MermaidBlockProps) => {
         // Generate a unique ID for this diagram
         const id = `mermaid-${Math.random().toString(36).substring(7)}`;
 
+        // Get the appropriate Mermaid theme for current app theme
+        const mermaidTheme = getMermaidTheme(currentTheme);
+
+        // Initialize mermaid with current theme
+        mermaid.initialize({
+          startOnLoad: false,
+          theme: mermaidTheme,
+          securityLevel: "strict",
+          fontFamily: "inherit",
+        });
+
         // Render the mermaid diagram
         const { svg: renderedSvg } = await mermaid.render(id, codeContent);
         setSvg(renderedSvg);
@@ -45,7 +93,7 @@ export const MermaidBlock = ({ children, className }: MermaidBlockProps) => {
     };
 
     renderDiagram();
-  }, [codeContent]);
+  }, [codeContent, currentTheme]);
 
   // If there's an error, fall back to showing the code
   if (error) {