fix(hub): merge duplicate sources in Search's Music and Photos too

Phases 5-8 of docs/refactoring-search.md, extending the Videos merge
outward. A library shared by two groups now lists each track once inside
an album and each photo once inside a photo album, and a card served by
several groups says "N sources" instead of naming one of them.

Units come from each application's own grouping, never a copy of its
keys. For Music that meant exporting foldKey: groupMusicEntries folds
case to group but keeps the first-seen spelling to display, and which
group is seen first is whichever index arrived first — so keying a unit
on the display strings would let the chosen source change between page
loads.

A group whose connection fails is marked down and stops being chosen, so
a unit fails over to another group that has the file. Eviction is not a
failure. Every source being down still yields an entry: a tile that
fails to load beats a film that vanished from the grid.

sourceLabel now takes the whole unit rather than one entry. A show's
poster entry is picked for its thumbnail, so a show in two groups whose
cover episode sits in only one of them would have claimed a single
source. SourceTag lives in group-name.js — source-merge.js must keep
importing nothing (its test executes it standalone), and a copy in each
of the three apps is three chances to disagree.

test_search_files_unmerged.py holds the one thing that must not change:
the Files explorer is not merged, because there each group is a folder
and merging would remove a file from one of them. It also asserts the
other three lists are merged, or deleting the merge outright would leave
it passing and saying nothing.

One plan item was dropped as wrong rather than built: the Music queue in
onPreview needed no change. It filters by groupId and is reachable only
from FilesPanel, which is not merged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AbwJDbNTkiRUh7HTWEoyss

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