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)

2 files changed, 99 insertions, 1 deletions

diff --git a/src/features/tabs/ui/CloseSubagentsConfirm.svelte b/src/features/tabs/ui/CloseSubagentsConfirm.svelte
new file mode 100644
index 0000000..cbbec79
--- /dev/null
+++ b/src/features/tabs/ui/CloseSubagentsConfirm.svelte
@@ -0,0 +1,57 @@
+<script lang="ts">
+  import type { Tab } from "../tabs";
+
+  /**
+   * The close-subagents confirm dialog: shown when the user closes a tab that
+   * still has subagent tabs in its (visible) subagent group. ONE dialog for
+   * the whole subtree — "Yes" cascades the close to every transitive
+   * descendant (sub-sub-agents included, no further prompts); "No" closes
+   * only the tab itself. Escape / backdrop dismisses without closing anything.
+   * daisyUI `modal` (controlled: `modal-open`).
+   */
+  let {
+    tab,
+    subagentCount,
+    onKeep,
+    onCloseAll,
+    onDismiss,
+  }: {
+    /** The tab being closed (drives the title shown in the dialog). */
+    tab: Tab;
+    /** Number of tabs in the tab's subagent group (transitive). */
+    subagentCount: number;
+    /** "No" — close only this tab, keep the subagents open. */
+    onKeep: () => void;
+    /** "Yes" — close this tab AND cascade to all its subagents. */
+    onCloseAll: () => void;
+    /** Escape / backdrop — close nothing. */
+    onDismiss: () => void;
+  } = $props();
+</script>
+
+<svelte:window onkeydown={(e) => e.key === "Escape" && onDismiss()} />
+
+<div class="modal modal-open" role="dialog" aria-label="Close subagents too?">
+  <div class="modal-box max-w-md">
+    <h3 class="text-lg font-bold">Close subagents too?</h3>
+    <div class="flex flex-col gap-1 py-3">
+      <p class="text-sm">
+        "{tab.title}" has {subagentCount}
+        {subagentCount === 1 ? "subagent tab" : "subagent tabs"} (including nested ones).
+      </p>
+      <p class="text-sm">
+        Close them as well? Their own subagents are closed too, and any in-flight turns are
+        stopped.
+      </p>
+    </div>
+    <div class="modal-action">
+      <button type="button" class="btn btn-ghost btn-sm" onclick={onDismiss}>Cancel</button>
+      <button type="button" class="btn btn-sm" onclick={onKeep}>No, keep them open</button>
+      <button type="button" class="btn btn-warning btn-sm" onclick={onCloseAll}>
+        Yes, close all {subagentCount + 1}
+      </button>
+    </div>
+  </div>
+  <!-- svelte-ignore a11y_click_events_have_key_events, a11y_no_static_element_interactions -->
+  <div class="modal-backdrop" onclick={onDismiss}></div>
+</div>
diff --git a/src/features/tabs/ui/TabList.svelte b/src/features/tabs/ui/TabList.svelte
index 06d5aa6..4b96e08 100644
--- a/src/features/tabs/ui/TabList.svelte
+++ b/src/features/tabs/ui/TabList.svelte
@@ -1,6 +1,7 @@
 <script lang="ts">
   import {
     buildTabGroups,
+    cascadeCloseIds,
     directGroupChildren,
     groupGutterPx,
     groupParentCandidates,
@@ -8,6 +9,7 @@
   } from "../tab-groups";
   import type { Tab } from "../tabs";
   import { shortHandle } from "../tabs";
+  import CloseSubagentsConfirm from "./CloseSubagentsConfirm.svelte";
   import TabCard from "./TabCard.svelte";
   import TabGroupBox from "./TabGroupBox.svelte";
   import TabMenu from "./TabMenu.svelte";
@@ -72,6 +74,26 @@
     if (onSetTabGroup === undefined) return;
     menu = { conversationId, x, y };
   }
+
+  // Close-with-confirm: a tab whose (visible) subagent group is non-empty
+  // gets ONE confirm dialog covering its whole subtree (never one per level).
+  // The dialog's ids are snapshotted at open time; a tab the backend closed
+  // in the meantime is a no-op for `onClose`. A plain tab closes directly.
+  let closeConfirm = $state<{ tab: Tab; cascadeIds: readonly string[] } | null>(null);
+
+  function requestClose(conversationId: string): void {
+    const cascadeIds = cascadeCloseIds(tabs, conversationId);
+    if (cascadeIds.length <= 1) {
+      onClose(conversationId);
+      return;
+    }
+    const tab = tabs.find((t) => t.conversationId === conversationId);
+    if (tab === undefined) {
+      onClose(conversationId);
+      return;
+    }
+    closeConfirm = { tab, cascadeIds };
+  }
 </script>
 
 {#snippet card(tab: Tab, wide: boolean, subagentCount: number)}
@@ -83,7 +105,7 @@
     {wide}
     {subagentCount}
     {onSelect}
-    {onClose}
+    onClose={requestClose}
     {onRename}
     onMenu={openMenu}
   />
@@ -147,3 +169,22 @@
     onDismiss={() => (menu = null)}
   />
 {/if}
+
+{#if closeConfirm !== null}
+  {@const target = closeConfirm.tab}
+  {@const cascadeIds = [...closeConfirm.cascadeIds]}
+  {@const closeOwner = onClose}
+  <CloseSubagentsConfirm
+    tab={target}
+    subagentCount={cascadeIds.length - 1}
+    onKeep={() => {
+      closeOwner(target.conversationId);
+      closeConfirm = null;
+    }}
+    onCloseAll={() => {
+      for (const id of cascadeIds) closeOwner(id);
+      closeConfirm = null;
+    }}
+    onDismiss={() => (closeConfirm = null)}
+  />
+{/if}