fix(dashboard): close SSE connections on outbound backpressure

The global SSE broadcast called res.write() without checking the return
value, so a paused or backgrounded client would silently accumulate
every store event for every entity (tasks, missions, plugins, agents,
chat, ...) into res.outputData until the dashboard process OOMed.
Add a 4 MB writableLength threshold; when exceeded, tear down the
connection so the OS releases the buffer.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

packages/dashboard/app/components/BackendConnectionErrorPage.tsx

@@ -0,0 +1,24 @@
+interface BackendConnectionErrorPageProps {
+  errorMessage: string;
+  isRetrying: boolean;
+  onRetry: () => void;
+}
+
+export function BackendConnectionErrorPage({
+  errorMessage,
+  isRetrying,
+  onRetry,
+}: BackendConnectionErrorPageProps) {
+  return (
+    <div className="project-overview-empty" role="alert" aria-live="polite">
+      <h2>Can&apos;t reach the Fusion backend</h2>
+      <p className="settings-muted">
+        Fusion couldn&apos;t load your projects right now. Please make sure the backend is running and try again.
+      </p>
+      <p className="settings-muted">Error: {errorMessage}</p>
+      <button type="button" className="btn btn-primary" onClick={onRetry} disabled={isRetrying}>
+        {isRetrying ? "Retrying…" : "Retry Connection"}
+      </button>
+    </div>
+  );
+}