fix(visual): stop fixture window from covering Linux bottom panels

[afonsojramos]

Summary

After #31 made the fixture window opaque white (to fix GNOME bleed-through), Budgie and Cinnamon started failing: their default panels are at the bottom of the screen, and our window snaps to the bottom-right corner (because tray.getBounds() returns {0,0,0,0} on Linux SNI, so menubar's Positioner falls back). The opaque white window now sits right on top of the tray icon → exactTray=0, test fails. KDE Plasma showed the same hang/failure pattern.

Previously the window was transparent (Electron default on Linux) so the tray showed through.

Fix: explicitly mb.window.setPosition(400, 200) after showWindow(). Mid-screen placement guarantees no overlap with any panel (top or bottom) on any platform. On macOS/Windows the visual diff just moves where the white box appears in the masked screenshot — relative position doesn't matter for diffing.

Test plan

• Budgie + Cinnamon now pass tray detection
• KDE Plasma completes in normal ~3min instead of hanging
• Other platforms unchanged