Fix DM presence indicator never updating live (#63)

The sidebar shows every DM's online/offline dot at once, but the only
existing signal for a presence change (member_updated) is broadcast to
a room's own channel, which Presence only delivers to a connection that
currently has that specific room joined -- never true for a DM sitting
unopened in the sidebar. Add a dedicated per-user broadcast
(dm_presence_update) sent to each of a user's DM partners on their own
per-user channel whenever their global online/offline state changes, so
the sidebar dot updates without needing that DM to be the open room.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/pages/ChatShellPage.tsx

@@ -77,6 +77,14 @@ export function ChatShellPage() {
                 : r,
             ),
           )
+        } else if (envelope.type === 'dm_presence_update') {
+          setRooms((prev) =>
+            prev.map((r) =>
+              r.dm_partner && r.dm_partner.user_id === envelope.user_id
+                ? { ...r, dm_partner: { ...r.dm_partner, status: envelope.status } }
+                : r,
+            ),
+          )
         }
       }),
     [socket, refreshRooms, refreshMembers, roomId],