Calendar reminders open the event; phones get the Calendar panel

A reminder notification links to its day, event and occurrence
(/p/calendar?date=…&event=…&start=…). The calendar opens on that day with the
event's details showing — the full calendar on a computer; on a phone any
calendar link leads to Mail with the Calendar panel slid in (the big-screen
page never opens there), scrolled to the event, details open.

The panel's agenda kept no scroll position across re-renders: opening an
event's details lower down jumped the list back to the top. It stays put now.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/tests/plugins/test_calendar_reminders.py

@@ -104,7 +104,7 @@ async def test_due_reminders_are_sent_once(ctx) -> None:  # type: ignore[no-unty
     assert await run(MON_9 - timedelta(minutes=60)) == 1  # the 1-hour reminder
     assert sent[-1]["title"] == "Dentist"
     assert sent[-1]["body"] == "In 1 hour, 09:00 · Main St 4"
-    assert sent[-1]["url"] == "/p/calendar?date=2031-06-02" and sent[-1]["sticky"] is True
+    assert sent[-1]["url"].startswith("/p/calendar?date=2031-06-02&event=") and sent[-1]["sticky"] is True
     assert await run(MON_9 - timedelta(minutes=59)) == 0  # not again
     # The worker was down when the 10-minute reminder came due: sent late, once.
     assert await run(MON_9 - timedelta(minutes=4)) == 1