feat(FN-4733): complete Step 4 — add chat cleanup settings UI

Fusion-Task-Id: FN-4733
Fusion-Task-Lineage: 38495ced-c7a0-40aa-be22-7f02fe958806

packages/dashboard/app/components/SettingsModal.tsx

@@ -2149,6 +2149,26 @@ export function SettingsModal({
               </label>
               <small>Show the floating chat button in the dashboard. Chat is still accessible from the Chat tab in the mobile navigation.</small>
             </div>
+            <h4 className="settings-section-heading settings-section-heading--spaced">Chat history</h4>
+            <div className="form-group">
+              <label htmlFor="chatAutoCleanupDays">Auto-cleanup old chats</label>
+              <select
+                id="chatAutoCleanupDays"
+                className="select"
+                value={form.chatAutoCleanupDays ?? 0}
+                onChange={(e) =>
+                  setForm((f) => ({ ...f, chatAutoCleanupDays: Number(e.target.value) || 0 }))
+                }
+              >
+                <option value={0}>Off</option>
+                <option value={7}>7 days</option>
+                <option value={14}>14 days</option>
+                <option value={30}>30 days</option>
+                <option value={60}>60 days</option>
+                <option value={90}>90 days</option>
+              </select>
+              <small>Delete chat sessions and rooms that have been idle for this many days. Default: Off.</small>
+            </div>
             <h4 className="settings-section-heading settings-section-heading--spaced">Chat Rooms</h4>
             <div className="form-group">
               <label htmlFor="chatRoomRecentVerbatimMessages">Recent verbatim room messages</label>

packages/dashboard/app/components/__tests__/SettingsModal.test.tsx

@@ -863,6 +863,24 @@ describe("SettingsModal", () => {
       expect(ephemeralToggle.checked).toBe(true);
     });
 
+    it("renders and saves chat auto-cleanup retention", async () => {
+      renderModal({ initialSection: "general" });
+      await waitForSettingsModalReady();
+
+      const cleanupSelect = screen.getByLabelText("Auto-cleanup old chats") as HTMLSelectElement;
+      expect(cleanupSelect.value).toBe("0");
+
+      await userEvent.selectOptions(cleanupSelect, "14");
+      await userEvent.click(screen.getByRole("button", { name: "Save" }));
+
+      await waitFor(() => {
+        expect(mockUpdateSettings).toHaveBeenCalled();
+      });
+
+      const payload = mockUpdateSettings.mock.calls[0][0] as Record<string, unknown>;
+      expect(payload.chatAutoCleanupDays).toBe(14);
+    });
+
     it("renders and saves chat room compaction controls", async () => {
       renderModal({ initialSection: "general" });
       await waitForSettingsModalReady();