fix(files): make "Filter files" search the whole group

The filter only ever matched files whose folder was exactly the current
path, so at the top of a group — where every row is a root, never a loose
file — typing in the field did nothing at all.

With text in the field the panel now searches every file in the group by
name or containing folder, wherever it lives, and hides the folder rows
(you are searching, not browsing). Each result shows its folder as a
sub-line; clicking it clears the filter and opens that folder. Empty field
restores the normal folder-by-folder view unchanged.

Also hardened the sort comparators against an entry missing `name`/`type`.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018gKJ85aZyvEwarXMFzFEwi

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