fix: resolve ERR_INSUFFICIENT_RESOURCES connection exhaustion

- Add concurrency limiter (max 4 GET requests) to API layer, leaving
  slots for SSE and health checks. Write ops bypass the limiter.
- Add AbortController to ContainerStats, project detail page, and
  dashboard to cancel in-flight requests on navigation/unmount.
- Move global SSE connection from layout to events page (only consumer).
- Add 30s heartbeat to SSE endpoint to detect zombie connections.
- Serialize dashboard project fetches to avoid parallel burst.
- Rebuild frontend in dev-server.sh so go:embed stays in sync.

scripts/dev-server.sh

@@ -32,6 +32,10 @@ export ENCRYPTION_KEY
 export ADMIN_PASSWORD="${ADMIN_PASSWORD:-admin123}"
 export LISTEN_ADDR="${PORT}"
 
+# Rebuild frontend so go:embed picks up changes.
+echo "Building frontend..."
+(cd web && npm run build --silent)
+
 echo "Starting Tinyforge on http://localhost:${PORT_NUM}"
 echo "Login: admin / ${ADMIN_PASSWORD}"
 exec go run ./cmd/server