feat(notes): push, long polling and fallback detection for note sync

Phase 4 of the collaboration plan:
- The sync engine listens for DOC_SEQ in the note's realtime room and syncs
  right away. Without WebSocket/SSE it long-polls /docs/{id}/changes (25 s);
  after three failures it falls back to polling every 3 s and retries long
  polling when the browser comes back online.
- With push or long polling the engine only syncs every 7 s while holding
  leases (renewal), otherwise every 30 s as a safety net. Focusing a block
  announces its lease immediately (otherwise it could take up to 30 s).
- Realtime connection: SSE counts as connected only after its first event;
  a stream that stays silent for 5 s (buffering proxy) switches SSE off for
  the tab. App-level PING/PONG detects WebSockets that are accepted but
  forward nothing and switches to SSE after 35 s.
- The editor shows the active channel for shared notes.
- Tests: engine channel tests (push, heartbeat, lease on focus, long
  polling, interval fallback, switching), realtime fallback tests with fake
  WebSocket/EventSource, Go tests for PONG and DOC_SEQ delivery, and
  e2e/channels.e2e.mjs measuring latency behind six simulated proxy setups
  (0.2-0.3 s on all live channels, ~3 s with plain polling).

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

e2e/run.sh

@@ -15,4 +15,4 @@ for _ in $(seq 1 60); do curl -s -m 1 http://localhost:18080/api/v1/health >/dev
 
 echo "Starte Browser-Tests …"
 "$ENGINE" run --rm --network host -v "$ROOT/e2e":/e2e:Z -w /e2e mcr.microsoft.com/playwright:v1.48.2-jammy \
-  sh -c 'npm install --no-audit --no-fund >/dev/null 2>&1 && node collab.e2e.mjs && node share.e2e.mjs && node lock.e2e.mjs'
+  sh -c 'npm install --no-audit --no-fund >/dev/null 2>&1 && node collab.e2e.mjs && node share.e2e.mjs && node lock.e2e.mjs && node channels.e2e.mjs'