Fix Split Canvas red-error flash and show background work in pane tabs

[ReganBell]

Master spec: nothing. The spec no longer lives in this repo.

Two web-UI fixes and one test-fixture cleanup.

A Split Canvas refresh paints red errors in healthy panes

boot() fetches /api/runtime-config to fill the model menus, then mounts a
chat — and the mount called refreshRuntimeSelection, which nulls
activeRuntimeConfig, redraws, and fetches the same URL again. For that whole
round trip the composer rendered its pending state, and the pending state used
.composer-error — color: var(--destructive). So a perfectly healthy mount
painted red for one request. In a Split Canvas each pane is its own SPA boot,
so the gap repeats per pane, staggered behind each pane's own
/me → runtime-config → sessions chain: some panes red at any instant, red
moving around, everything clean once the last one lands. Nothing was failing.

Hand boot's config to the composer and let the first mount in that scope render
from it. Render a load still in flight as .composer-note, keeping the error's
red and its Retry for a real failure.

The same refresh also read as the page loading twice: boot() mounted the
canvas after await refreshSessions(...), but a pane's iframe src comes from
the persisted layout, not the list — every pane was waiting on a request none
of them reads. Mount on the bare-URL entry before the fetch instead. That hands
the user an interactive canvas while the fetch is still in flight, so the
fallback is now gated on nothing being mounted: closing down to one pane in
that window runs reconcileAfterClose, and the previously unguarded
newChat() replaced the very conversation the user had just been handed.

Measured against a dev build with injected API latency: on a 12-pane refresh,
twelve red banners spread over 5.4s become zero and /api/runtime-config drops
from 25 requests to 13. On a 4-pane canvas, frames start at 0.9s instead of
1.3–4.0s and the canvas settles in 5.5s instead of 6.0–8.6s.

Show a conversation's background work in its pane tab

The Recents row and the Chats page badge a conversation that has detached work
parked in it. A pane's tab is that conversation's header once the pane is
stacked behind a tab, and it showed only the working and awaiting dots — so
work running in a tabbed-away conversation was invisible until you clicked the
tab. The tab now renders the same chip through conversationBackground, and
computeHeaderSignature counts the background label so a tab redraws when work
starts or ends instead of holding a stale mark.

Before / after, captured against synthetic data:

Neutral names in the mention-arming fixtures

test/slack-mrkdwn.test.ts seeded its name index with real people's first
names, full names and a real surname. Placeholder identities instead; every
case is unchanged.

Verification

npm test (3553 pass, 0 fail), plugins/web-ui tests (439 pass, 0 fail),
typecheck, lint and format:check all clean.

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