feat(ui): add light theme toggle

src/App.test.tsx

@@ -18,4 +18,23 @@ describe("React app dashboard state view", () => {
     expect(html).toContain("Loading Dashboard");
     expect(html).toContain("Fetching active model");
   });
+
+  test("renders an accessible theme toggle with the active theme", () => {
+    const html = renderToString(
+      <AppStateView
+        dashboard={{
+          state: "loading",
+          title: "Loading Dashboard",
+          subtitle: "Fetching active model",
+          message: "Waiting for the active dashboard document.",
+        }}
+        theme="dark"
+        onThemeChange={() => undefined}
+      />,
+    );
+
+    expect(html).toContain('data-ui-theme-toggle="true"');
+    expect(html).toContain('aria-label="Switch to light theme"');
+    expect(html).toContain("Dark");
+  });
 });