docs: document side-panel drag-to-resize and SharedWorker SSE multiplexing (follow-up to #451 #453)

docs/web-ui.md

@@ -49,6 +49,16 @@ and the per-agent UIs (manager on :8000, sub-agents on a hashed
   ~200 broker messages wrapped in `{ seq, events }`) on the
   dashboard and `GET /events/history` (last 2000 `LiveEvent`s
   also wrapped in `{ seq, events }`) on the agent.
+  **SSE multiplexing** (#448): the dashboard uses a
+  `SharedWorker` (`stream-worker.js`) to hold one upstream
+  `EventSource` per URL. All same-origin tabs share this worker
+  — a second dashboard tab joins the existing connection rather
+  than opening a duplicate. The worker fans SSE events out to
+  each subscribed tab via `MessagePort`; on bfcache restore the
+  page re-subscribes (gets a synthetic `open` event immediately
+  if the upstream is already connected). Falls back gracefully
+  when `SharedWorker` is unavailable (e.g. some private-mode
+  browsers).
 
 The JS app handles all `form[data-async]` submissions via a delegated
 listener: read `data-confirm`, swap the button to a spinner, POST
@@ -79,7 +89,15 @@ swipes in from the right — a singleton `#side-panel` with a
 titled header, a close button, and a scrollable body. Closes on
 the button, a backdrop click, or `Escape`. `Panel.open(title,
 node)` swaps the body; the JS builders for file previews,
-approval diffs, and journald logs all render into it. File
+approval diffs, and journald logs all render into it. **The
+drawer width is drag-to-resize** (#451): a thin 6px hit-strip on
+the left edge captures pointer events, resizes the drawer in
+real-time (pointer capture keeps dragging even if the cursor
+outpaces the handle), and persists the chosen width to
+`localStorage` (key `hyperhive:side-panel-width`) so it
+survives page reload. Width is clamped to CSS `min-width: 320px`
+/ `max-width: 96vw`; the viewport-resize handler re-clamps
+persisted values after a window shrink. File
 previews are type-aware:
 
 - **Markdown** (`.md` / `.markdown`) — a `rendered` / `plain`