Improve API error messages: show HTTP status, catch network errors

"Unknown error" was hiding the actual HTTP status (likely 502 from
nginx). Now shows "HTTP 502 Bad Gateway" etc. Network TypeErrors
(connection refused) also get a clear message.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

apps/web/src/lib/api.ts

@@ -86,8 +86,8 @@ async function request<T>(path: string, options: RequestInit & { timeoutMs?: num
     });
 
     if (!res.ok) {
-      const body = await res.json().catch(() => ({ error: 'Unknown error' }));
+      const body = await res.json().catch(() => null);
-      throw new Error(body.error || `HTTP ${res.status}`);
+      throw new Error(body?.error || `HTTP ${res.status} ${res.statusText}`);
     }
 
     return res.json();
@@ -95,6 +95,9 @@ async function request<T>(path: string, options: RequestInit & { timeoutMs?: num
     if (e instanceof DOMException && e.name === 'AbortError') {
       throw new Error('Request timed out — server may be unreachable');
     }
+    if (e instanceof TypeError) {
+      throw new Error('Network error — server may be unreachable');
+    }
     throw e;
   } finally {
     clearTimeout(timeout);