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)