fix: memory leak in SSE log accumulation and infinite reload loop in NodeContext

- Cap GlobalObservabilityView SSE log array at 10,000 entries to prevent
  unbounded memory growth during long dev-mode sessions
- Break NodeContext infinite re-fetch loop by replacing the activeNode
  closure dependency in refreshNodes with a useRef read, making the
  callback stable and preventing the useEffect -> setState -> useCallback
  -> useEffect cycle

frontend/src/components/GlobalObservabilityView.tsx

@@ -93,9 +93,8 @@ export function GlobalObservabilityView() {
                     const batch = bufferRef.current.splice(0);
                     setLogs(prev => {
                         const merged = [...prev, ...batch];
-                        // Infinite scroll: no slice limit in dev mode
                         merged.sort((a, b) => a.timestampMs - b.timestampMs);
-                        return merged;
+                        return merged.slice(-10000);
                     });
                 }
             }, 500);