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/static/css/sneakyswole.css

@@ -34,3 +34,36 @@ main.container {
     padding: 0.5rem 1rem;
     margin-bottom: 1rem;
 }
+
+/* Muscle group heatmap */
+.muscle-heatmap {
+    display: grid;
+    grid-template-columns: repeat(auto-fill, minmax(150px, 1fr));
+    gap: 0.75rem;
+}
+
+.muscle-heatmap-cell {
+    padding: 1rem;
+    border-radius: 0.5rem;
+    text-align: center;
+}
+
+.recency-fresh {
+    background: rgba(34, 197, 94, 0.3);
+}
+
+.recency-ok {
+    background: rgba(234, 179, 8, 0.3);
+}
+
+.recency-stale {
+    background: rgba(249, 115, 22, 0.3);
+}
+
+.recency-overdue {
+    background: rgba(239, 68, 68, 0.3);
+}
+
+.recency-never {
+    background: rgba(107, 114, 128, 0.3);
+}