fix(electron): show the main window once its document has loaded

The hidden main window was shown on ready-to-show only. On Linux under
X11, when the startup scripts run before the window's first frame, the
next frame comes about a second later: nothing is on screen and the
splash's requestAnimationFrame waits, so J1's first card landed ~450 ms
late in about a third of the CI launches (18 bridge calls / 1018 DOM
mutations instead of 15 / 559).

The window is now shown at ready-to-show or the main frame's
did-finish-load, whichever comes first, with the splash colour as its
background so showing before the first paint does not flash. The journey
gate keeps the app's did-finish-load listeners away from its about:blank
detour, as it already does for ready-to-show.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.changes/electron-window-shown-on-load.md

@@ -0,0 +1,8 @@
+---
+type: perf
+area: electron
+---
+
+On Linux the desktop app's window no longer sometimes appears about a second
+late at launch: it now opens as soon as the app has loaded, showing the
+loading screen until the dashboard is ready.