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, 1 insertions, 0 deletions

diff --git a/src/features/tabs/index.ts b/src/features/tabs/index.ts
index 980e8ee..0094528 100644
--- a/src/features/tabs/index.ts
+++ b/src/features/tabs/index.ts
@@ -1,6 +1,7 @@
 export type { TabGroupForest, TabGroupNode } from "./tab-groups";
 export {
   buildTabGroups,
+  cascadeCloseIds,
   directGroupChildren,
   groupBleedPx,
   groupDescendants,