Add ESLint for TypeScript/React linting

- Install eslint, typescript-eslint, eslint-plugin-react-hooks
- Configure eslint.config.js with flat config format
- Add type: module to package.json
- Fix unused variable issues (prefix with underscore)
- Add Makefile targets: lint-frontend, fix-frontend

frontend/e2e/profile.spec.ts

@@ -357,7 +357,7 @@ test.describe("Profile - Unauthenticated Access", () => {
     await expect(page).toHaveURL("/login");
   });
 
-  test("profile API requires authentication", async ({ page, request }) => {
+  test("profile API requires authentication", async ({ page: _page, request }) => {
     const response = await request.get(`${API_URL}/api/profile`);
     expect(response.status()).toBe(401);
   });