feat: show subsessions under parent sessions in mobile sheet

Nest child sessions (by parentID) under their parent in the mobile sessions
list, with a chevron in the row's left gutter to expand/collapse them
recursively. Top-level pagination counts only parent sessions; children whose
parent isn't in the same bucket stay top-level so nothing is hidden.

Expansion state lives in an in-memory store that survives closing and
reopening the sheet but resets on a full page reload.

packages/ui/src/stores/useMobileSessionExpansionStore.ts

@@ -0,0 +1,23 @@
+import { create } from 'zustand';
+
+/**
+ * In-memory expand state for parent sessions (with subsessions) in the mobile
+ * sessions sheet. Deliberately NOT persisted: it survives closing and
+ * reopening the sheet (the sheet unmounts on close, but this module-level store
+ * does not), and resets on a full page reload.
+ */
+type MobileSessionExpansionStore = {
+  expandedParents: Record<string, boolean>;
+  toggleParent: (sessionId: string) => void;
+};
+
+export const useMobileSessionExpansionStore = create<MobileSessionExpansionStore>((set) => ({
+  expandedParents: {},
+  toggleParent: (sessionId) =>
+    set((state) => {
+      const next = { ...state.expandedParents };
+      if (next[sessionId]) delete next[sessionId];
+      else next[sessionId] = true;
+      return { expandedParents: next };
+    }),
+}));