fix(desktop): read error body instead of statusText for gateway errors

res.statusText is always empty under HTTP/2, causing "Gateway error: "
with no detail. Now reads the JSON response body to surface the
server's actual error message and always includes the status code.

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

apps/desktop/src/renderer/src/components/telegram-qr.tsx

@@ -50,11 +50,9 @@ export function TelegramConnectQR({
         if (cancelled) return
 
         if (!res.ok) {
-          if (res.status === 503) {
-            setError('Telegram bot not configured on Gateway')
-          } else {
-            setError(`Gateway error: ${res.statusText}`)
-          }
+          const body = (await res.json().catch(() => null)) as { message?: string } | null
+          const detail = body?.message || `HTTP ${res.status}`
+          setError(`Gateway error (${res.status}): ${detail}`)
           setDeepLink(null)
           return
         }