fix: blank terminal when switching cached sessions

Two bugs introduced by the terminal instance cache that caused blank terminals:

Bug 1 (primary — every second+ visit to a cached session):
  switchTerminal()'s section 4 calls createTerminal(), which disposes the
  existing _term before creating a new one.  At the time section 4 runs,
  _term still points to the PREVIOUSLY-ACTIVE session's terminal — the one
  sitting live in its own cache entry.  After disposal the cache entry holds
  a dead xterm instance; the next visit to that session via the fast-path
  calls fit()/focus() on the disposed terminal, both silently no-op, and the
  terminal renders blank.

  Fix: null out _term and _fitAddon in section 4's 'Reset module-level state'
  block BEFORE createTerminal(), so the previously-active session's terminal
  is left untouched in its cache entry.

Bug 2 (secondary — WS dies while session is in background):
  When a cached session's WebSocket closes while hidden, the close handler
  bails early via the stale-guard (ws !== _ws), because _ws now points to the
  active session's socket.  No reconnect is scheduled.  On revisit:
  isTerminalCached() returns false → openSession() POSTs /connect → but
  switchTerminal() takes the cache-HIT branch (_terminalCache.has() is still
  true), shows the container, and returns — without creating a new WebSocket.
  Terminal container appears but has no live connection → blank.

  Fix: in the cache-HIT branch, check WS readyState before taking the fast
  path.  If the socket is CLOSED or CLOSING, call destroyCachedEntry() and
  fall through to section 4 so a fresh terminal + connection is created.
  (openSession() has already POST'd /connect, so ttyd is ready.)

muxplex/frontend/terminal.js

@@ -661,9 +661,14 @@ function switchTerminal(sessionName, remoteId, fontSize) {
     }
   }
 
-  // 3. If target is in cache — instant switch, no network
+  // 3. If target is in cache — check WebSocket liveness before the fast path
   if (_terminalCache.has(key)) {
     var entry = _terminalCache.get(key);
+    var wsIsLive = entry.ws &&
+                   (entry.ws.readyState === WebSocket.OPEN ||
+                    entry.ws.readyState === WebSocket.CONNECTING);
+    if (wsIsLive) {
+      // Fast path: live connection — swap containers, no network round-trip
       if (entry.container) {
         entry.container.style.display = '';
       }
@@ -676,6 +681,13 @@ function switchTerminal(sessionName, remoteId, fontSize) {
       if (_term) _term.focus();
       return;
     }
+    // WebSocket died while this session was in the background.
+    // The close handler's stale-guard (ws !== _ws) suppresses reconnects for
+    // non-active sessions, so the entry is left with a dead socket.
+    // Destroy the stale entry and fall through to create a fresh terminal.
+    // openSession() already POST'd /connect so the backend ttyd is ready.
+    destroyCachedEntry(key);
+  }
 
   // 4. Not in cache — create new entry
   // Evict LRU if at capacity
@@ -698,7 +710,12 @@ function switchTerminal(sessionName, remoteId, fontSize) {
   subContainer.style.height = '100%';
   parentContainer.appendChild(subContainer);
 
-  // Reset module-level state for fresh creation
+  // Reset module-level state for fresh creation.
+  // Null _term/_fitAddon BEFORE createTerminal() so it does NOT dispose the
+  // previously-active session's terminal — that terminal lives on in its own
+  // cache entry and must stay alive for instant switching later.
+  _term = null;
+  _fitAddon = null;
   _currentSession = null;
   _reconnectAttempts = 0;
   _reconnectTimer = null;