fix: key messages by networkId:room to prevent collision across networks

Two networks sharing a room name (e.g. "general") were clobbering each
other's message lists. Now keyed by `${networkId}:${room}` throughout
the web store, MessagePane, and Sidebar DM detection.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/components/MessagePane.tsx

@@ -16,8 +16,9 @@ export function MessagePane() {
   const { messages, historyCutoff, activeRoom, activeNetworkId, localPeer, connectedPeers, knownPeers, send } = useWaste()
   const [draft, setDraft] = useState('')
   const bottomRef = useRef<HTMLDivElement>(null)
-  const roomMessages = messages[activeRoom] ?? []
-  const cutoff = historyCutoff[activeRoom] ?? 0
+  const msgKey = activeNetworkId ? `${activeNetworkId}:${activeRoom}` : activeRoom
+  const roomMessages = messages[msgKey] ?? []
+  const cutoff = historyCutoff[msgKey] ?? 0
 
   // Find the index of the first live message (ts > cutoff).
   // The divider appears just before this index, or at the top if all are history.