fix(browse): mirror isCustomChromium() guard in headless launch()

When BROWSE_EXTENSIONS_DIR is set alongside GSTACK_CHROMIUM_PATH pointing
at a baked-extension build (GBrowser / GStack Browser), the headless launch()
path was unconditionally adding --disable-extensions-except / --load-extension.
This causes the same ServiceWorkerState::SetWorkerId DCHECK crash that
launchHeaded() already guards against via isCustomChromium().

Mirror the existing guard: skip --load-extension flags when isCustomChromium()
returns true; always push the off-screen window geometry args.

browse/src/browser-manager.ts

@@ -307,12 +307,16 @@ export class BrowserManager {
     }
 
     if (extensionsDir) {
-      launchArgs.push(
-        `--disable-extensions-except=${extensionsDir}`,
-        `--load-extension=${extensionsDir}`,
-        '--window-position=-9999,-9999',
-        '--window-size=1,1',
-      );
+      // Skip --load-extension when running against a custom Chromium build that
+      // already bakes the extension in (e.g., GBrowser / GStack Browser.app).
+      // Loading it twice causes a ServiceWorkerState::SetWorkerId DCHECK crash.
+      if (!isCustomChromium()) {
+        launchArgs.push(
+          `--disable-extensions-except=${extensionsDir}`,
+          `--load-extension=${extensionsDir}`,
+        );
+      }
+      launchArgs.push('--window-position=-9999,-9999', '--window-size=1,1');
       useHeadless = false; // extensions require headed mode; off-screen window simulates headless
       console.log(`[browse] Extensions loaded from: ${extensionsDir}`);
     }