feat: add CSV export of workout history to dashboard

Add date range picker and download button to the dashboard page,
backed by GET /dashboard/export endpoint that returns a StreamingResponse
CSV file. Completes Phase 4 (final V2 improvement).

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

app/templates/pages/dashboard.html

@@ -42,5 +42,8 @@
         {% endfor %}
     </ul>
 </article>
+
+<!-- Export -->
+{% include "partials/export_form.html" %}
 {% endif %}
 {% endblock %}