feat: Add dismissible warning UI for threshold loading errors

- Replace console.warn with visible MessageBar warning when map color thresholds fail to load
- Add DismissRegular icon button to allow users to dismiss the warning
- Add dismissedThresholdWarning state to manage warning visibility
- Add mock and test for useMapColorThresholds hook
- Add test case verifying warning displays and can be dismissed
- Remove TASK-QUALITY-04 from Tasks.md (completed)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

frontend/src/pages/MapPage.tsx

@@ -10,6 +10,7 @@ import { useState, useMemo, useEffect } from "react";
 import {
   Button,
   MessageBar,
+  MessageBarActions,
   MessageBarBody,
   Spinner,
   Text,
@@ -142,12 +143,7 @@ export function MapPage(): React.JSX.Element {
   const thresholdMedium = mapThresholds?.threshold_medium ?? 50;
   const thresholdHigh = mapThresholds?.threshold_high ?? 100;
 
-  useEffect(() => {
-    if (mapThresholdError) {
-      // Silently fall back to defaults if fetch fails
-      console.warn("Failed to load map color thresholds:", mapThresholdError);
-    }
-  }, [mapThresholdError]);
+  const [dismissedThresholdWarning, setDismissedThresholdWarning] = useState(false);
 
   useEffect(() => {
     setPage(1);
@@ -223,6 +219,24 @@ export function MapPage(): React.JSX.Element {
         </MessageBar>
       )}
 
+      {mapThresholdError && !dismissedThresholdWarning && (
+        <MessageBar intent="warning">
+          <MessageBarBody>
+            Map color thresholds could not be loaded. Using default thresholds.
+          </MessageBarBody>
+          <MessageBarActions>
+            <Button
+              appearance="transparent"
+              size="small"
+              icon={<DismissRegular />}
+              onClick={(): void => {
+                setDismissedThresholdWarning(true);
+              }}
+            />
+          </MessageBarActions>
+        </MessageBar>
+      )}
+
       {/* Initial load spinner — only shown before the first data arrives. */}
       {loading && !error && !hasLoadedOnce && (
         <div className={styles.initialLoad}>

frontend/src/pages/__tests__/MapPage.test.tsx

@@ -31,6 +31,20 @@ vi.mock("../../hooks/useMapData", () => ({
   },
 }));
 
+let thresholdState: { thresholds: unknown; error: string | null } = {
+  thresholds: { threshold_low: 10, threshold_medium: 50, threshold_high: 100 },
+  error: null,
+};
+
+vi.mock("../../hooks/useMapColorThresholds", () => ({
+  useMapColorThresholds: () => ({
+    ...thresholdState,
+    loading: false,
+    refresh: () => {},
+    updateThresholds: () => {},
+  }),
+}));
+
 vi.mock("../../api/config", () => ({
   fetchMapColorThresholds: vi.fn(() => ({
     threshold_low: 10,
@@ -113,4 +127,36 @@ describe("MapPage", () => {
     expect(getLastArgs().origin).toBe("blocklist");
     expect(await screen.findByText(/Page 1 of 5/i)).toBeInTheDocument();
   });
-});
+
+  it("displays warning when map color thresholds fail to load", async () => {
+    const user = userEvent.setup();
+
+    setMapData({
+      countries: { US: 10 },
+      countryNames: { US: "United States" },
+      bans: [],
+      total: 0,
+      loading: false,
+      error: null,
+    });
+
+    thresholdState = {
+      thresholds: null,
+      error: "Failed to fetch thresholds",
+    };
+
+    render(
+      <FluentProvider theme={webLightTheme}>
+        <MapPage />
+      </FluentProvider>,
+    );
+
+    expect(screen.getByText(/Map color thresholds could not be loaded/i)).toBeInTheDocument();
+    expect(screen.getByText(/Using default thresholds/i)).toBeInTheDocument();
+
+    // User can dismiss the warning
+    const dismissButton = screen.getByRole("button", { name: "" });
+    await user.click(dismissButton);
+    expect(screen.queryByText(/Map color thresholds could not be loaded/i)).not.toBeInTheDocument();
+  });
+})