Kill the GraphRAG flash: warm the view up, and never paint it white

Two causes, both dealt with.

A fresh QWebEngineView paints white, and _ensure_web put it on screen empty —
so on a dark theme that white rectangle sat there for the whole first scan.
It is now blanked to the app's own background colour the moment it is created,
before it is ever shown.

And the work itself moved off the click. MainWindow warms GraphRAG up 3s after
the window appears — building the browser view (~140ms) and the first graph
(~485ms) while nothing is waiting on them. Clicking GraphRAG then costs 78ms
with zero scans and zero setHtml calls, where before it was ~625ms of empty
view.

This spends the memory the lazy construction was saving, a few seconds after
startup rather than never — which is the trade you asked for. Startup itself is
untouched: the checker asserts neither the view nor the graph exists at the
moment the window opens.

check_graphrag_rescan now covers the warm-up too, and fails if it stops running.

24/24 checkers pass; check_combo_popup took a teardown segfault in the suite and
passed 3/3 standalone.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

app.py

@@ -416,6 +416,21 @@ class MainWindow(QMainWindow):
             self._update_dock_guard()
             self.help_agent.reposition()
             self.help_agent.raise_()
+        # Build GraphRAG's browser view and first graph once the window is up
+        # and idle, so clicking GraphRAG does not sit on an empty view while
+        # both happen. 3s is after the first paint and any startup refresh.
+        if not getattr(self, "_graph_prewarmed", False):
+            self._graph_prewarmed = True
+            QTimer.singleShot(3000, self._prewarm_graph)
+
+    def _prewarm_graph(self) -> None:
+        view = getattr(self, "structure", None)
+        if view is None or not hasattr(view, "prewarm"):
+            return
+        try:
+            view.prewarm()
+        except Exception:  # noqa: BLE001 — a warm-up must never break the app
+            pass
 
     # ---- i18n ----------------------------------------------------------
     def _retranslate(self) -> None: