Add --disable-gpu flag to Playwright browser launch

Stampli dashboard hangs in the Playwright-managed Chromium due to the
GPU process (SwiftShader software renderer) pegging CPU at ~135%.
Disabling GPU acceleration works around the issue but does not fully
resolve the hang — further investigation needed (see issue).

stampli_core.py

@@ -87,6 +87,7 @@ def launch_browser(pw):
         user_data_dir=str(BROWSER_DATA_DIR),
         headless=False,
         viewport={"width": 1400, "height": 900},
+        args=["--disable-gpu"],
     )