feat(web): Add reconnecting feedback when SSE connection is lost (#125)

- Add ReconnectingBanner component to show "Reconnecting..." status
- Track SSE connection state in App.tsx with onDisconnect handler
- Add onBlocked callback to useSendMessage for handling blocked send attempts
- Provide haptic feedback when send is blocked due to no API or session
- Show toast notification when message cannot be sent due to server disconnection
- Add translation keys for reconnecting message and send blocked feedback in en/zh-CN

close #125

web/src/components/ReconnectingBanner.tsx

@@ -0,0 +1,19 @@
+import { useOnlineStatus } from '@/hooks/useOnlineStatus'
+import { useTranslation } from '@/lib/use-translation'
+
+export function ReconnectingBanner({ isReconnecting }: { isReconnecting: boolean }) {
+    const { t } = useTranslation()
+    const isOnline = useOnlineStatus()
+
+    // Don't show if offline (OfflineBanner takes precedence) or if not reconnecting
+    if (!isReconnecting || !isOnline) {
+        return null
+    }
+
+    return (
+        <div className="fixed top-0 left-0 right-0 bg-amber-500 text-white text-center py-2 text-sm font-medium z-50 flex items-center justify-center gap-2">
+            <span className="animate-spin h-4 w-4 border-2 border-white border-t-transparent rounded-full" />
+            {t('reconnecting.message')}
+        </div>
+    )
+}