feat: copy a file's or folder's #/name@owner link from Files, Music, Photos and Search

"Copy link" puts the address group-link.js resolves on the clipboard, on the
hub's origin rather than the page's, so a link copied in the desktop
application is not app://meshbay. Files offers it for one row, from the
right-click menu or the toolbar with one row ticked (a phone's way in);
Music on one track's menu, whose dots a phone has; Photos on a right-clicked
tile and in the lightbox's bar. The video player and the file preview carry
a link button next to Download.

Applications get a `linkFor(entry | folderPath)` prop (MESHBAY_DESIGN.md
§9.2) and offer the action only when it names a link. The group page builds
it from the hub's row; Search from each result's own group and its path
before the merged views prefixed it, and names no link for a folder of the
merged tree, which a group name alone does not identify.

harness/copy_link_probe.py mounts the three applications in Chrome and reads
what reached the clipboard.

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

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