fix(frontend): show a friendly message when the backend is unreachable

request()'s fetch() call throwing (network down, connection refused,
offline) previously propagated the raw browser error text (e.g.
"TypeError: Failed to fetch"). Centralizing the friendly message here,
rather than in each call site, means every existing
catch((e) => showError(String(e))) pattern gets it for free.

frontend/src/api/client.ts

@@ -16,11 +16,22 @@ import type {
 export const BASE_URL = import.meta.env.VITE_API_BASE_URL ?? "http://localhost:8080";
 
 async function request<T>(path: string, init?: RequestInit): Promise<T> {
-  const res = await fetch(`${BASE_URL}${path}`, {
+  let res: Response;
+  try {
+    res = await fetch(`${BASE_URL}${path}`, {
       headers: { "Content-Type": "application/json" },
       credentials: "include",
       ...init,
     });
+  } catch {
+    // fetch() itself rejecting (not a non-2xx response, which is handled
+    // below) means the backend genuinely isn't reachable -- offline,
+    // connection refused, CORS, etc. The raw browser error text here
+    // (e.g. "TypeError: Failed to fetch") is not something to show a user;
+    // every caller's catch-and-showError already displays whatever this
+    // throws, so the friendly text only needs to be written once, here.
+    throw new Error("Can't reach the server — check your connection.");
+  }
   if (res.status === 401 && path !== "/api/session/me") {
     // An established session expired mid-use (not the initial "am I logged
     // in" check, which LoginGate itself interprets) -- reload so LoginGate