fix(ui): harden theme toggle accessibility

tests/e2e/dashboard.spec.ts

@@ -240,6 +240,24 @@ test.describe("dashboard page QA gate", () => {
     expect(results.violations).toEqual([]);
   });
 
+  test("passes automated accessibility checks in light mode", async ({
+    page,
+  }, testInfo) => {
+    test.skip(testInfo.project.name !== "chromium-desktop");
+
+    await page.goto("/");
+    await waitForDashboardReady(page);
+    await page.getByRole("button", { name: "Switch to light theme" }).click();
+    await expect(page.locator("html")).toHaveAttribute("data-ui-theme", "light");
+
+    const results = await new AxeBuilder({ page }).analyze();
+    expect(results.violations).toEqual([]);
+
+    await expect(page).toHaveScreenshot("dashboard-light-desktop.png", {
+      fullPage: true,
+    });
+  });
+
   test("honors reduced-motion preferences", async ({ page }) => {
     await page.emulateMedia({ reducedMotion: "reduce" });