feat(web): click Executions' Started time to toggle relative/absolute

Both renderings are pre-rendered server-side into a <button>'s
data-relative/data-absolute attributes; app.js swaps textContent (and the
hover title, to the opposite value) on click, delegated on document.body
so it survives the card's own 5s htmx poll without needing to re-wire
after every swap. Click again to swap back.

tests/python/test_web_executions.py

@@ -340,6 +340,34 @@ def test_status_badge_has_no_icon_for_non_terminal_states(
     assert "#i-x" not in resp.text
 
 
+def test_started_cell_is_a_click_to_toggle_button(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """Both the relative and absolute renderings are pre-rendered into the
+    button's data attributes -- app.js swaps textContent/title on click, no
+    round trip. Server-side, this test can only pin the markup the toggle
+    depends on, not the click itself."""
+    _stub_view(
+        monkeypatch,
+        ExecutionsView(
+            runs=[_execution(started="2026-07-25T11:50:00Z")],
+            unmatched_vms=[],
+            source="admin",
+            error=None,
+        ),
+    )
+    app = create_app(_make_config(tmp_path))
+    app.dependency_overrides[get_backend] = lambda: _FakeBackend([])
+    client = TestClient(app)
+
+    resp = client.get("/executions")
+
+    assert '<button type="button" class="time-toggle text-dim"' in resp.text
+    assert 'data-showing="relative"' in resp.text
+    assert 'data-relative="' in resp.text
+    assert 'data-absolute="' in resp.text
+
+
 def test_no_cell_stacks_two_facts_on_separate_lines(
     tmp_path: Path, monkeypatch: pytest.MonkeyPatch
 ) -> None: