experimental: pop-out container terminals into dedicated windows

Pin PTY sessions to the peer they were opened on so shells survive
active-node switches until the window is closed. Electron host owns
the connection; pop-out windows relay I/O via IPC (BroadcastChannel
fallback). Explicit terminal sessionIds no longer wipe sibling PTYs
on the same container.

server/handlers/terminal.js

@@ -412,13 +412,22 @@ export function registerTerminalHandlers(session) {
     const sessions = getSessions(session)
     const sessionId = resolveSessionId(args)
 
-    // Always clear any prior PTY for this id/container before opening a new one.
-    // Prevents races where a late killTerminal and a re-entry startTerminal overlap,
-    // and avoids "stuck" sessions when the client re-enters the Terminal tab.
+    // Replace any prior PTY for this exact session id (re-entry / restart).
+    // Explicit unique sessionIds (details-*, popout-*) may coexist on the same
+    // container so pop-out windows stay alive while the in-app tab reopens.
     if (sessions.has(sessionId)) {
       endOne(sessions, sessionId)
     }
-    cleanupTerminalsForContainer(session, containerId)
+    // Legacy clients key the session by containerId only — clear leftovers for
+    // that container so re-open is clean. Explicit multi-session ids skip this.
+    const explicitSession =
+      args.sessionId != null &&
+      String(args.sessionId) !== '' &&
+      String(args.sessionId) !== String(containerId) &&
+      String(args.sessionId) !== 'default'
+    if (!explicitSession) {
+      cleanupTerminalsForContainer(session, containerId)
+    }
 
     const useTty = args.tty !== false
     const shellCandidates = buildShellCandidates(args)