feat(ui): an indexing dock above the music bar, on every page

Adding a large directory left the operator nothing to look at once they left
the Settings panel that started it, and nothing at all when it was added from
another machine. A band now sits above the music bar on every page: one row
per group with indexing under way, naming the root being walked, percent,
bytes and files, and the roots waiting their turn; "indexing finished" for a
few seconds at the end. A click opens the group's Settings, and × hides the
row until that group is idle.

Two sources feed it. On the node's own machine the desktop client polls the
loopback `GET /api/index-status` for every group, whatever the route. An
operator's group page forwards MNP `index_progress` pushes, resolving the
root from the roots table it opened; an ordinary member keeps the sidebar dot
only, and a page clears its row when it lets go of the group. Where both
describe a group, loopback wins.

Reconcile passes and watchdog bursts show only past 1 GB or 5 s, so a single
dropped file does not flash a bar. The logic lives in index-dock-model.js,
which has no imports and is tested under node. The dock publishes
`--index-dock-h` and the sidebar stops above it and the music bar.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T6jPTeocXA1BePekdsgPya

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