chat: "Disconnect session" — end the whole session from the UI (first stab)

Adds a per-session teardown, distinct from the existing turn-only "stop":
- New WS 'disconnect' message → handleDisconnect → sessionManager.deleteSession,
  which fires _claudeKill (kills any in-flight Claude/OpenCode turn) + _sidecarUnsub,
  clears the idle timer, and drops the in-memory session. WS stays open so a new
  prompt starts fresh. Server acks with 'disconnected'.
- useChat: disconnectSession() + a 'disconnected' handler (commit partial stream,
  settle to idle).
- UI: an Unplug button in the chat DetailBar (shown while connected).

Scope note: targets the CURRENTLY-OPEN session (correct in-memory sessionKey).
Disconnecting an arbitrary *listed* session isn't wired yet — session-list rows are
keyed by the on-disk transcript uuid, which isn't the live sessionKey, so that needs
a reverse lookup + a REST endpoint. NOT yet deployed (needs a server restart).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/servers/api/chat/websocket.ts

@@ -120,6 +120,8 @@ export function message(ws: ServerWebSocket<WSData>, raw: string | Buffer): void
         await handleResume(ws, clientMsg);
       } else if (clientMsg.type === 'stop') {
         await handleStop(ws);
+      } else if (clientMsg.type === 'disconnect') {
+        await handleDisconnect(ws);
       }
     } catch (err) {
       logger.error('Error handling WebSocket message', { email: ws.data.email, error: String(err) });
@@ -538,6 +540,23 @@ async function handleStop(ws: ServerWebSocket<WSData>): Promise<void> {
   sendToClient(ws, { type: 'stopped' });
 }
 
+// Tear down the whole session (not just the current turn): deleteSession fires _claudeKill (kills any
+// in-flight Claude/OpenCode turn) + _sidecarUnsub, clears the idle timer, and drops the session from the
+// manager's maps. The WS stays open so the client can immediately start a fresh session.
+async function handleDisconnect(ws: ServerWebSocket<WSData>): Promise<void> {
+  const sessionId = wsToSessionMap.get(ws);
+  if (sessionId) {
+    try {
+      sessionManager.deleteSession(sessionId);
+      wsToSessionMap.delete(ws as any);
+      logger.info('Disconnected chat session', { sessionId });
+    } catch (err) {
+      logger.error('Failed to disconnect session', { sessionId, error: String(err) });
+    }
+  }
+  sendToClient(ws, { type: 'disconnected' });
+}
+
 export const chatWebsocket = {
   open,
   message,