fix(visual): make sway respect window size and stop GNOME painting it blue

[afonsojramos]

Summary

Two cosmetic fixes for the residual platforms after the white-box marker landed:

Sway: window tiled to fill the workspace

Sway is a tiling compositor and ignores Electron's 200x100 size request, stretching the fixture to fill all available space. Added a for_window rule to the test's Sway config that matches by the HTML <title> ("menubar visual fixture") and forces the window floating + 200x100 + position (400, 200). Same dimensions and position as every other platform now.

GNOME: window paints navy-blue instead of white

Mutter was either painting a CSD header bar accent in our window region, or rendering it as "unfocused" with a dimming tint — either way our backgroundColor: '#FFFFFF' wasn't visible. Three changes to the fixture's browserWindow opts:

• frame: false: no CSD header bar, no Mutter accent strip inside our 200x100 rect.
• useContentSize: true: 200x100 is now the content size; without this, the inner HTML viewport was smaller than 200x100 because Electron sized the outer rect.
• mb.window.focus() after setPosition: stops Mutter rendering the window with its "unfocused window" tint.

Test plan

• Sway screenshot shows a 200x100 white box with the 80x40 black inner square (not a fullscreen white area)
• GNOME screenshot shows white background + black square (not navy-blue rectangle)
• Other platforms unchanged