feat: enhance dashboard with PRs, adherence, activity, progression chart, and muscle heatmap

Add 3 new stat cards (Last Workout, Personal Records, Adherence Rate),
recent activity table, progression timeline chart, and muscle group
recency heatmap to the dashboard. Remove Total Volume card.

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

app/templates/partials/recent_activity.html

@@ -0,0 +1,29 @@
+<article>
+    <header><h3>Recent Activity</h3></header>
+    {% if recent_activity %}
+    <div style="overflow-x:auto;">
+        <table>
+            <thead>
+                <tr>
+                    <th>Date</th>
+                    <th>Workout</th>
+                    <th>Volume (lbs)</th>
+                    <th>Sets</th>
+                </tr>
+            </thead>
+            <tbody>
+                {% for session in recent_activity %}
+                <tr>
+                    <td>{{ session.date.strftime('%b %d, %Y') }}</td>
+                    <td>{{ session.workout_day_name }}</td>
+                    <td>{{ "{:,}".format(session.total_volume) }}</td>
+                    <td>{{ session.total_sets }}</td>
+                </tr>
+                {% endfor %}
+            </tbody>
+        </table>
+    </div>
+    {% else %}
+    <p>No workout sessions logged yet.</p>
+    {% endif %}
+</article>