Review findings (minor) on #120912:
* `AppearanceExtraSlot` wrapped each contribution in `ContribBoundary
variant="chip"`, so a page-level plugin card that threw collapsed into a
bar-item chip meant for toolbar slots. Use the default `pane` variant —
the canonical ErrorState with Retry, matching every other zone body.
* The slot was mounted unconditionally, so every deep-link subpage
(`settings/appearance/<section>`, which shows exactly one built-in
section) also grew the plugin cards. Gate it on `subpage === undefined`
like the rest of the page's top-level-only chrome.
Tests: the boundary test now asserts the pane fallback (red on c322f692);
one new page-level test renders `AppearanceSettings` with and without a
subpage and asserts the extra mounts only on the top-level page (red on
c322f692). Docs updated to the new contract.
55c52f2902