feat: SessionList.set_mode swaps header for archived view

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_tui.py

@@ -2015,3 +2015,23 @@ async def test_h_focuses_projects_l_focuses_sessions(tmp_path):
         await pilot.press("h")
         await pilot.pause()
         assert app.focused is app.query_one("#project-list")
+
+
+@pytest.mark.asyncio
+async def test_session_list_header_reflects_archived_mode():
+    app = HqtApp()
+    async with app.run_test(size=(120, 40)) as pilot:
+        from hqt.tui.widgets.session_list import SessionList
+        from textual.widgets import Label
+
+        sl = app.query_one(SessionList)
+        header = sl.query_one("#session-header", Label)
+        assert str(header.render()) == "Sessions"
+
+        sl.set_mode(archived=True)
+        await pilot.pause()
+        assert "archived" in str(header.render()).lower()
+
+        sl.set_mode(archived=False)
+        await pilot.pause()
+        assert str(header.render()) == "Sessions"