fix(web): Started toggle survives the Executions card's 5s poll

The card's own htmx poll re-renders the whole table from the server every
5s, always starting fresh at "relative" -- so a toggled-to-absolute cell
snapped back on the next poll. Each button now has a stable id
(started-{run_id}-{attempt}, or started-vm-{vmx} on an unmatched-VM row);
app.js remembers which format was showing per id and re-applies it to the
fresh button after every swap, rather than freezing the node itself (which
would also freeze its relative-time text, badge, duration, everything else
in that row).

tests/python/test_web_executions.py

@@ -346,11 +346,17 @@ def test_started_cell_is_a_click_to_toggle_button(
     """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."""
+    depends on, not the click itself.
+
+    The id must be stable across the card's own 5s poll (run_id/attempt, not
+    anything generated fresh per render) -- app.js keys its client-side
+    "which format is showing" memory off it to survive the next poll's
+    server-rendered reset back to relative.
+    """
     _stub_view(
         monkeypatch,
         ExecutionsView(
-            runs=[_execution(started="2026-07-25T11:50:00Z")],
+            runs=[_execution(run_id=412, attempt=1, started="2026-07-25T11:50:00Z")],
             unmatched_vms=[],
             source="admin",
             error=None,
@@ -362,7 +368,8 @@ def test_started_cell_is_a_click_to_toggle_button(
 
     resp = client.get("/executions")
 
-    assert '<button type="button" class="time-toggle text-dim"' in resp.text
+    assert 'id="started-412-1"' in resp.text
+    assert '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