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);

apps/web/src/lib/hooks/useWorkspaceId.test.ts

@@ -0,0 +1,109 @@
+/**
+ * useWorkspaceId Hook Tests
+ * Tests for SEC-WEB-35: warning logging when workspace ID is not found
+ */
+
+import { describe, it, expect, vi, beforeEach, afterEach } from "vitest";
+import { renderHook } from "@testing-library/react";
+import { useWorkspaceId } from "./useLayout";
+
+interface MockLocalStorage {
+  getItem: ReturnType<typeof vi.fn>;
+  setItem: ReturnType<typeof vi.fn>;
+  removeItem: ReturnType<typeof vi.fn>;
+  clear: ReturnType<typeof vi.fn>;
+  readonly length: number;
+  key: ReturnType<typeof vi.fn>;
+}
+
+// Mock localStorage
+const localStorageMock = ((): MockLocalStorage => {
+  let store: Record<string, string> = {};
+  return {
+    getItem: vi.fn((key: string): string | null => store[key] ?? null),
+    setItem: vi.fn((key: string, value: string): void => {
+      store[key] = value;
+    }),
+    removeItem: vi.fn((key: string): void => {
+      store = Object.fromEntries(Object.entries(store).filter(([k]) => k !== key));
+    }),
+    clear: vi.fn((): void => {
+      store = {};
+    }),
+    get length(): number {
+      return Object.keys(store).length;
+    },
+    key: vi.fn((_index: number): string | null => null),
+  };
+})();
+
+Object.defineProperty(window, "localStorage", {
+  value: localStorageMock,
+  writable: true,
+});
+
+describe("useWorkspaceId", (): void => {
+  let consoleWarnSpy: ReturnType<typeof vi.spyOn>;
+  let consoleErrorSpy: ReturnType<typeof vi.spyOn>;
+
+  beforeEach((): void => {
+    vi.clearAllMocks();
+    localStorageMock.clear();
+    consoleWarnSpy = vi.spyOn(console, "warn").mockImplementation(() => undefined);
+    consoleErrorSpy = vi.spyOn(console, "error").mockImplementation(() => undefined);
+  });
+
+  afterEach((): void => {
+    consoleWarnSpy.mockRestore();
+    consoleErrorSpy.mockRestore();
+    vi.resetAllMocks();
+  });
+
+  it("should return workspace ID when stored in localStorage", (): void => {
+    localStorageMock.setItem("mosaic-workspace-id", "ws-123");
+
+    const { result } = renderHook(() => useWorkspaceId());
+
+    expect(result.current).toBe("ws-123");
+    expect(consoleWarnSpy).not.toHaveBeenCalled();
+  });
+
+  it("should return null and log warning when no workspace ID in localStorage", (): void => {
+    const { result } = renderHook(() => useWorkspaceId());
+
+    expect(result.current).toBeNull();
+    expect(consoleWarnSpy).toHaveBeenCalledTimes(1);
+    expect(consoleWarnSpy).toHaveBeenCalledWith(
+      expect.stringContaining("No workspace ID found in localStorage")
+    );
+  });
+
+  it("should include the storage key in the warning message", (): void => {
+    renderHook(() => useWorkspaceId());
+
+    expect(consoleWarnSpy).toHaveBeenCalledWith(expect.stringContaining("mosaic-workspace-id"));
+  });
+
+  it("should log console.error when localStorage throws", (): void => {
+    localStorageMock.getItem.mockImplementation(() => {
+      throw new Error("localStorage is disabled");
+    });
+
+    const { result } = renderHook(() => useWorkspaceId());
+
+    expect(result.current).toBeNull();
+    expect(consoleErrorSpy).toHaveBeenCalledWith(
+      "Failed to load workspace ID from localStorage:",
+      expect.any(Error)
+    );
+  });
+
+  it("should not log warning when workspace ID exists", (): void => {
+    localStorageMock.setItem("mosaic-workspace-id", "ws-abc-def");
+
+    renderHook(() => useWorkspaceId());
+
+    expect(consoleWarnSpy).not.toHaveBeenCalled();
+    expect(consoleErrorSpy).not.toHaveBeenCalled();
+  });
+});