feat(core): show real per-result icons in unified search

[pringelmann]

• Resolves: Result presentation: cap per category, "More from …", detail view #61652

Summary

Reworks how unified search presents results, per the design in #61652.

The aggregate list now shows at most three rows per category, each group headed by a "More from …" control that opens that category's full result page in a detail view. Result rows show their provider's real icon instead of a generic one.

Smaller fixes from the same pass:

• Refining a query keeps the previous results on screen while the new ones load,
  instead of flashing an empty panel.
• The header only takes space when it has something to show, so the detail view
  and the resting empty state no longer carry a blank padded strip.
• A second Ctrl+F falls through to the browser's find bar.
• Directional icons and the input's resting state flip correctly in RTL.

SectionSearch now uses each settings section's own icon. That reverses an older workaround whose comment noted those icons did not invert for dark theme; the new AppIcon component handles the inversion, so the icon URL is safe to use again.

Design feedback already captured

The following feedback items were already captured. They will land in a follow-up PR: I want to get the core work merged, then we iterate.

• We do show "Ctrl K" as shortcut, but the more commonly known (?) shortcut Ctrl-F also works. Would it not be better to show that so people know they can use their muscle memory for that? (For the few people who already use that F shortcut)
• The shortcut looks veeery prominent visually if it’s there all the time. Might be good to only show on hover or so?
• When focusing/clicking into the field, suggestions of any kind would be good. Right now the quick filters are hidden behind the "filter" icon on the right instead of being shown, and we could show recent searches (or suggested results)
• Result sorting is a bit odd. Here in files I have a matching filename, but an app/setting is shown before?
  Also see screenshot: the PDF filetype icon is too small and also blue
• "Apps" is sorted first instead of current app?
• Detail: Double horizontal line on "Partial matches"
• When I'm diving into a result type where the top filters could apply, they should be available to further refine the search at a later step. for instance when I dive into files, I would still keep "date" and "people" around

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)
• Labels added where applicable (ex: bug/enhancement, 3. to review, feature component)
• Milestone added for target branch/version (ex: 32.x for stable32)

AI (if applicable)

• The content of this PR was partly or fully generated using AI (mostly tests)