feat: ExerciseDB integration with muscle heatmap, SVG body filter, and enriched exercises

Integrate ExerciseDB v2 data layer (muscleMap.ts, exercisedb.ts) to enrich
the 77 static exercises with detailed muscle targeting, similar exercises,
and expand the catalog to 254 exercises. Add interactive SVG muscle body
diagrams for both the stats page heatmap and exercise list filtering, with
split front/back views flanking the exercise list on desktop. Replace body
part dropdown with unified muscle group multi-select with pill tags.

src/routes/fitness/[stats=fitnessStats]/+page.svelte

@@ -1,6 +1,7 @@
 <script>
 	import { page } from '$app/stores';
 	import FitnessChart from '$lib/components/fitness/FitnessChart.svelte';
+	import MuscleHeatmap from '$lib/components/fitness/MuscleHeatmap.svelte';
 	import { Dumbbell, Route, Flame, Weight } from '@lucide/svelte';
 	import FitnessStreakAura from '$lib/components/fitness/FitnessStreakAura.svelte';
 	import { onMount } from 'svelte';
@@ -221,6 +222,11 @@
 			height="220px"
 		/>
 	{/if}
+
+	<div class="section-block">
+		<h2 class="section-title">{t('muscle_balance', lang)}</h2>
+		<MuscleHeatmap data={data.muscleHeatmap} />
+	</div>
 </div>
 
 <style>
@@ -519,4 +525,16 @@
 		color: var(--color-text-secondary);
 		padding: 2rem 0;
 	}
+
+	.section-block {
+		background: var(--color-surface);
+		border-radius: 12px;
+		padding: 1rem;
+		box-shadow: var(--shadow-sm);
+	}
+	.section-title {
+		margin: 0 0 0.75rem;
+		font-size: 1rem;
+		font-weight: 700;
+	}
 </style>