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>

frontend/src/features/workspace/DocumentEditor.tsx

@@ -687,7 +687,7 @@ export const DocumentEditor: React.FC<DocumentEditorProps> = ({
   }
 
   return (
-    <div className="flex-1 flex flex-col h-full min-h-0 bg-canvas overflow-y-auto">
+    <div className="flex-1 flex flex-col h-full min-h-0 bg-canvas overflow-y-auto" data-sync-channel={engineState?.channel}>
       {/* Top Document Action Bar */}
       <div className="sticky top-0 z-20 flex flex-wrap items-center justify-between gap-2 px-4 sm:px-6 py-2 bg-canvas border-b border-border">
         <div className="flex items-center gap-2 min-w-0">
@@ -766,6 +766,29 @@ export const DocumentEditor: React.FC<DocumentEditorProps> = ({
 
         {/* Right Action Buttons */}
         <div className="flex items-center gap-1.5">
+          {/* How changes of others arrive (only relevant for shared notes) */}
+          {engineState && (engineState.shared || engineState.sessions > 1) && (
+            <span
+              className="hidden sm:inline-flex items-center gap-1 px-2 py-0.5 rounded-btn text-[11px] text-muted bg-panel border border-border"
+              title={
+                engineState.channel === 'ws'
+                  ? 'Live-Verbindung über WebSocket'
+                  : engineState.channel === 'sse'
+                  ? 'Live-Verbindung über Server-Sent Events (WebSocket wird blockiert)'
+                  : engineState.channel === 'longpoll'
+                  ? 'Live über Long-Polling (WebSocket und SSE werden blockiert)'
+                  : 'Keine Live-Verbindung möglich – Änderungen werden alle 3 Sekunden abgefragt'
+              }
+            >
+              <span
+                className={`w-1.5 h-1.5 rounded-full ${
+                  engineState.channel === 'interval' ? 'bg-status-warn' : 'bg-status-ok'
+                }`}
+              />
+              {engineState.channel === 'longpoll' ? 'Live (Long-Polling)' : engineState.channel === 'interval' ? 'Abgleich alle 3 s' : 'Live'}
+            </span>
+          )}
+
           {/* Who else has this note open */}
           {otherPeople.length > 0 && (
             <span