refactor(frontend): lint, one session guard, one request helper, one user context

src/test/fetchRoutes.js

@@ -0,0 +1,20 @@
+import {vi} from 'vitest';
+
+export const json = (status, body) => ({ok: status >= 200 && status < 300, status, json: async () => body});
+
+/**
+ * A fetch mock that answers by method and path, e.g. `'GET /api/users/me'`. The path starts at `/api/` (the base URL
+ * is not set in tests) and leaves out the query. A value is an answer or `(url, init) => answer`. Any other request
+ * throws, so a test cannot pass on a request it did not expect.
+ * Sets `globalThis.fetch` and returns the mock.
+ */
+export function fetchRoutes(routes) {
+    const mock = vi.fn(async (url, init) => {
+        const key = `${init?.method ?? 'GET'} ${new URL(url, 'http://x').pathname.replace(/^.*?(?=\/api\/)/, '')}`;
+        if (!(key in routes)) throw new Error(`no route ${key}`);
+        const answer = routes[key];
+        return typeof answer === 'function' ? answer(url, init) : answer;
+    });
+    globalThis.fetch = mock;
+    return mock;
+}