fix: zombie terminals eating the quota; tab drop double-firing into two panes

- The per-session terminal quota counted exited zombie handles (shells
  closed or dropped on hosts without the close-frame protocol), so enough
  abandoned terminals permanently blocked new ones — the 'disconnected,
  reconnecting' loop. open() now sweeps the session's exited handles before
  the quota check, and the running instance's leftovers disappear on the
  next restart (disposeAll).
- Dragging a tab onto the strip's background fired the drop TWICE: the
  TabBar handled it (merge into this pane) and the event bubbled to the
  pane root, which ran its edge-zone handler again — producing two splits /
  a duplicated instance (up+down). The strip now stops propagation for
  drag-over and drop.
- Tests: +1 real-pty smoke (exited handle swept, new tab spawns under a
  quota of 1).

src/client/TabBar.tsx

@@ -64,12 +64,17 @@ export function TabBar(props: {
     <div
       className={clsx(css.tabBar, dragOver && css.tabBarDrop)}
       onDragOver={(event) => {
+        // The strip owns drops on itself (merge into this pane); stopping
+        // propagation keeps the pane root from also running its edge-zone
+        // handler on the same drop.
         event.preventDefault()
+        event.stopPropagation()
         setDragOver(true)
       }}
       onDragLeave={() => { setDragOver(false) }}
       onDrop={(event) => {
         event.preventDefault()
+        event.stopPropagation()
         setDragOver(false)
         const raw = event.dataTransfer.getData(TAB_DRAG_TYPE)
         const payload = parseDrag(raw)

src/pty-manager.ts

@@ -96,6 +96,11 @@ export class PtyManager {
     const existing = this.sessions.get(key)
     if (existing !== undefined && !existing.exited) return existing
     if (existing !== undefined) this.close(key)
+    // Zombie cleanup: a session's exited handles (shell closed, tab dropped
+    // on an old host without the close frame) must not eat the quota.
+    for (const [candidate, handle] of [...this.sessions]) {
+      if (handle.sessionId === sessionId && handle.exited) this.close(candidate)
+    }
     if (this.keysOf(sessionId).length >= this.maxPerSession) {
       throw new SidebarError('pty-error', `terminal limit reached (${this.maxPerSession}) for this session`, 400)
     }