fix(web): render SSE connection-status indicator (TASK-2027)

[xarmian]

What

From the PLAN-1984 Web-UX audit: the SSE connection status was already computed in sseService but never rendered, so a stale board gave no hint the realtime stream was down or unauthorized.

This renders a small, unobtrusive live/reconnecting/offline indicator on the collection board view, wired off the existing sseService.status state (no new polling / no new connection layer).

Changes

• New web/src/lib/components/SSEStatusIndicator.svelte — dot + label badge reading sseService.status. Reuses the visual idiom of the item page's collab-state badge:
  • connected → subtle "Live" (muted label, quiet green dot)
  • reconnecting → yellow "Reconnecting…" (pulsing dot; respects prefers-reduced-motion)
  • disconnected → red "Offline"
  • unauthorized → red "Disconnected" (session lost workspace access)
• [collection]/+page.svelte — renders the indicator in the header title-row (wrapped h1 + indicator in a .title-group so space-between layout stays clean).
• sse.svelte.ts — added a one-shot status_request BroadcastChannel handshake so a follower tab that opens after the leader already connected initializes its status correctly (BroadcastChannel doesn't replay the leader's earlier status message; without this a peer tab receiving live updates would wrongly show "Offline"). Found in Codex review.

Test plan

• cd web && npm run check — 0 errors on touched files.
• cd web && npm run build — succeeds.
• Codex review: CLEAN.
• Manual: open a collection board; indicator shows "Live". Kill the server → "Reconnecting…" then "Offline". Open a second tab while connected → follower initializes to "Live" via the handshake.

Refs TASK-2027, PLAN-1984.