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 %}

app/templates/partials/export_form.html

@@ -0,0 +1,19 @@
+<article>
+    <header><h3>Export Workout History</h3></header>
+    <form method="get" action="/dashboard/export">
+        <div class="grid">
+            <label>
+                Start Date
+                <input type="date" name="start_date" value="{{ export_start_date }}">
+            </label>
+            <label>
+                End Date
+                <input type="date" name="end_date" value="{{ export_end_date }}">
+            </label>
+            <label>
+                &nbsp;
+                <button type="submit">Download CSV</button>
+            </label>
+        </div>
+    </form>
+</article>