* gui: preview schemes as miniature windows and themes as orbs

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

* docs: specify the Preferences previews

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

* gui: outline previews in high contrast and keep them left-to-right

Review fixes: high-contrast edges for previews and orbs, LTR preview direction to match its CSS, the input bar as its own part (no specificity tie) with touching halves, and an accurate swatch comment.

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

* gui: split the System preview evenly and pin every part left to right

The two content halves sized from their natural widths, so the light
half ran 7px past the calc(21px + 50%) background and drew a dark
step and hairline at the seam. A homogeneous box now shares the space
equally. GTK does not inherit text direction, so the side card and
accent dot still flipped in RTL; every part is pinned LTR. The GTK
test asserts both.

* gui: keep press feedback on the checked appearance card

Dropping libadwaita's checked tint also dropped the pressed state of
an already selected card; restore the neutral 8% press tint.

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Co-authored-by: Lucenx9 <185146821+Lucenx9@users.noreply.github.com>
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>