feat(tabs): confirm dialog that cascades a tab close to its subagents

Closing a tab whose visible subagent group is non-empty now asks ONE
daisyUI modal (CloseSubagentsConfirm): 'Yes, close all N' closes the tab
plus every transitive descendant tab (sub-sub-agents included, no further
prompts), 'No, keep them open' closes only the tab, and Cancel/Escape/
backdrop closes nothing. Plain tabs close directly as before.

- cascadeCloseIds (pure, tab-groups.ts): the tab + its transitive visible
  group descendants in tab order — one decision covers the whole subtree
- TabList intercepts tab-close and opens the dialog only when the
  subtree has >1 tab; ids are snapshotted at open time
- each closed conversation goes through the existing onClose port, so
  every close still POSTs /conversations/:id/close (backend doesn't
  cascade; it is idempotent)

1 files changed, 16 insertions, 0 deletions

diff --git a/src/features/tabs/tab-groups.ts b/src/features/tabs/tab-groups.ts
index 12870ee..a1aa464 100644
--- a/src/features/tabs/tab-groups.ts
+++ b/src/features/tabs/tab-groups.ts
@@ -104,6 +104,22 @@ export function groupDescendants(tabs: readonly Tab[], conversationId: string):
 }
 
 /**
+ * The tabs removed by a CASCADING close of `conversationId` (the user chose
+ * "also close its subagents"): the tab itself first, then its transitive
+ * visible-group descendants in tab order. ONE decision covers the whole
+ * subtree — closing never re-prompts per level. A single-element result (the
+ * tab alone) means the tab has no subagents and closes without a prompt.
+ */
+export function cascadeCloseIds(tabs: readonly Tab[], conversationId: string): string[] {
+  const descendants = groupDescendants(tabs, conversationId);
+  const ids = [conversationId];
+  for (const tab of tabs) {
+    if (descendants.has(tab.conversationId)) ids.push(tab.conversationId);
+  }
+  return ids;
+}
+
+/**
  * The tabs `conversationId` may be moved under (the "Add to group" menu):
  * every other tab EXCEPT itself, its descendants (that would form a cycle),
  * and its current visible group parent (already there). Tab order preserved.