fix(engine): remove 4096MB GPU budget ceiling for high-RAM systems

Scale GPU budget to half of total RAM (capped at 16GB) instead of
hardcoding 4096MB. A 32GB machine now gets 16GB GPU budget; a 64GB
machine gets 16GB (Chrome's practical limit). Low-memory tiers unchanged.

packages/engine/src/services/browserManager.ts

@@ -484,7 +484,7 @@ function getGpuMemBudgetMb(): number {
   const total = getTotalMemMb();
   if (total < 4096) return 512;
   if (total < 8192) return 1024;
-  return 4096;
+  return Math.min(Math.floor(total / 2), 16384);
 }
 
 function getLowMemoryFlags(): string[] {