fitness: fix type errors, hydration warning, and add gym link

- Add exerciseId to WorkoutSession model (interface + schema)
- Fix button-in-button hydration warning in TemplateCard (use div)
- Expand FitnessChart dataset type to include all Chart.js properties
- Fix getTime type error in session update with proper cast
- Fix weight nullable type in profile stats with non-null assertion
- Fix $or query typing in templates list endpoint
- Re-add gym link on homepage pointing to /fitness

src/lib/components/fitness/FitnessChart.svelte

@@ -5,7 +5,7 @@
 	/**
 	 * @type {{
 	 *   type?: 'line' | 'bar',
-	 *   data: { labels: string[], datasets: Array<{ label: string, data: number[], borderColor?: string, backgroundColor?: string }> },
+	 *   data: { labels: string[], datasets: Array<{ label: string, data: (number|null)[], borderColor?: string, backgroundColor?: string, borderWidth?: number, pointRadius?: number, pointBackgroundColor?: string, tension?: number, fill?: boolean|string, order?: number }> },
 	 *   title?: string,
 	 *   height?: string,
 	 *   yUnit?: string

src/lib/components/fitness/TemplateCard.svelte

@@ -25,7 +25,7 @@
 	}
 </script>
 
-<button class="template-card" onclick={() => onStart?.()}>
+<div class="template-card" role="button" tabindex="0" onclick={() => onStart?.()} onkeydown={(e) => { if (e.key === 'Enter' || e.key === ' ') { e.preventDefault(); onStart?.(); }}}>
 	<div class="card-header">
 		<h3 class="card-title">{template.name}</h3>
 		{#if onMenu}
@@ -50,7 +50,7 @@
 	{#if lastUsed}
 		<p class="last-used">Last performed: {formatDate(lastUsed)}</p>
 	{/if}
-</button>
+</div>
 
 <style>
 	.template-card {