Show a message when the calendar has no events (#35)

## Summary
Closes #34.
- Upcoming and Past calendar tabs show a short message when that list is empty, so a calendar with no events is no longer a blank content area.
- Empty copy stays hidden on a tab that has events.

## Test plan
- [ ] Open `/calendar` with no `CalendarEvent` rows and confirm Upcoming says “There are no upcoming events to display.”
- [ ] Switch to Past and confirm it says “There are no past events to display.”
- [ ] Add an upcoming event and confirm the Upcoming message is gone while Past still shows its empty message if there are no past events.
- [ ] `uv run python manage.py test schasite.tests.PublicPageTests.test_calendar_shows_empty_state_when_no_events schasite.tests.CalendarEventTests.test_calendar_page_splits_past_and_future`

Reviewed-on: #35

schasite/templates/schasite/calendar2.html

@@ -136,6 +136,12 @@
                         </div>
                     </div>
                     
+                    {% empty %}
+                    <div class="col-12">
+                        <div class="alert alert-light border text-center mb-0" role="status">
+                            There are no upcoming events to display.
+                        </div>
+                    </div>
                     {% endfor %}
                 </div>
             </div>
@@ -179,9 +185,13 @@
                                 </div>
                             </div>
                         </div>
+                    {% empty %}
+                    <div class="col-12">
+                        <div class="alert alert-light border text-center mb-0" role="status">
+                            There are no past events to display.
+                        </div>
+                    </div>
                     {% endfor %}
-                     
-                    
                 </div>
             </div>
         </div>