feat(web): red border on a server card whose connection attempt failed (#1621)

Flags the server whose last connection attempt failed with a red card
border, cleared when another server is connected or a new connection is
attempted — a persistent, at-a-glance companion to the failure toast and
the auto-opened monitoring column.

- App tracks `failedServerId`: set in onToggleConnection's non-auth
  handshake catch, cleared at the start of any new connect attempt. It is
  deliberately kept out of resetSessionScopedUiState, since the failure's
  own `disconnect` event would otherwise clear it the same tick (that same
  event clears `activeServer`, which is why the flag is tracked separately).
- New `errored` Card theme variant (red border via a new
  `--inspector-error-border` token), mirroring the `highlighted` green
  border. Precedence: disabled (dimmed) > errored > highlighted.
- Plumbed `erroredServerId` through InspectorView → ServerListScreen →
  ServerCard (`errored` prop).

Full `npm run ci` passes (validate + coverage + smoke + storybook).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BrihGWcrM9JGRyu41nzZYw

clients/web/src/App.css

@@ -53,6 +53,10 @@
   /* Green border drawn around a freshly-added/highlighted server card (#1535) */
   --inspector-highlight-border: var(--mantine-color-green-6);
 
+  /* Red border drawn around a server card whose last connection attempt failed,
+     until another server is connected/attempted (#1621) */
+  --inspector-error-border: var(--mantine-color-red-6);
+
   /* ── Log levels (RFC 5424) ─────────────────────────────── */
   --inspector-log-debug: var(--mantine-color-gray-6);
   --inspector-log-info: var(--mantine-color-blue-6);