desktop: settings search, and an About landing page

The landing page's tiles repeated the sidebar. It now says what the shell
is and where things live, which is what a landing page is for.

The sidebar gains a search. A page is a Component loaded only when
opened, so its contents cannot be indexed without loading every page;
instead a module lists keywords for the settings buried in its page,
and the search matches those and the label. An entry matched by a
keyword shows which one, Enter opens the first match, Escape clears and
then steps out as before.

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

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