fix(e2e): run Qt headlessly via offscreen platform; drop xvfb

The e2e harness only needs mediasrv's HTTP stack — Playwright's own
Chromium connects to it directly. The Anki Qt window never needs to
be visible. Setting QT_QPA_PLATFORM=offscreen lets Qt initialise
without a display server, eliminating the exit-250 crash that happened
when Qt couldn't connect to the (virtual) framebuffer.

Also add PYTHONUNBUFFERED=1 so subprocess output reaches Playwright's
[WebServer] log immediately, and drop xvfb-run from the CI step since
it is no longer required.

Author: andrewsanchez

.github/workflows/ci.yml

@@ -115,7 +115,7 @@ jobs:
           fi
 
       - name: Run e2e tests
-        run: xvfb-run --auto-servernum just test-e2e
+        run: just test-e2e
 
       - name: Ensure libs importable
         env:

qt/tests/launch_anki_for_e2e.py

@@ -99,6 +99,12 @@ def main() -> int:
             "ANKIDEV": "1",
             "PYTHONPYCACHEPREFIX": str(REPO_ROOT / "out" / "pycache"),
             "RUST_BACKTRACE": "1",
+            # Headless Qt: the e2e harness only needs mediasrv's HTTP stack,
+            # not a visible window. The offscreen platform plugin renders to
+            # memory and requires no display server.
+            "QT_QPA_PLATFORM": "offscreen",
+            # Flush Python output immediately so Playwright captures it.
+            "PYTHONUNBUFFERED": "1",
         }
         env.pop("QTWEBENGINE_REMOTE_DEBUGGING", None)
         env.pop("QTWEBENGINE_CHROMIUM_FLAGS", None)