fix(settings): stop the Themes list jumping when accessibility reads it

[datlechin]

Why

testThemesListScrollsAndTakesArrowKeys failed on main and on PRs: the CI recordings show the list reaching the bottom, then snapping back. The list's table sizes a row it has not measured at 24 pt; a theme row is 42 pt. An accessibility read (XCUITest, VoiceOver) drops off-screen rows back to that estimate and remeasures them, so the rows above the visible ones change height and the list jumps.

Change

• The Themes list gives its table a 42 pt row estimate, so the rows stay put while VoiceOver or a UI test reads the list.
• The empty Custom placeholder row is as tall as a theme row.

Verified

AppearancePaneListTests reads the list through the AX API while scrolling. It fails on main ("Rows {14, 5} moved to {9, 6} while nothing scrolled") and passes here.

Not in this PR

• A Not Loaded row whose reason wraps past one line is taller than 42 pt and can still shift by the difference.