fix(SEC-WEB-33+35): Fix Mermaid error display + useWorkspaceId error logging

SEC-WEB-33: Replace raw diagram source and detailed error messages in
MermaidViewer error UI with a generic "Diagram rendering failed" message.
Detailed errors are logged to console.error for debugging only.

SEC-WEB-35: Add console.warn in useWorkspaceId when no workspace ID is
found in localStorage, making it easier to distinguish "no workspace
selected" from silent hook failure.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/lib/hooks/useLayout.ts

@@ -232,6 +232,11 @@ export function useWorkspaceId(): string | null {
       const stored = localStorage.getItem(WORKSPACE_KEY);
       if (stored) {
         setWorkspaceId(stored);
+      } else {
+        console.warn(
+          `useWorkspaceId: No workspace ID found in localStorage (key: "${WORKSPACE_KEY}"). ` +
+            "This may indicate no workspace has been selected yet."
+        );
       }
     } catch (error) {
       console.error("Failed to load workspace ID from localStorage:", error);