fix(planner): format variety score to one decimal place

Avoids floating-point display like 6.199999999999999 by using
score.toFixed(1) in VarietyScoreCard.

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

frontend/src/lib/planner/VarietyScoreCard.svelte

@@ -20,7 +20,7 @@
 <div class="rounded-[var(--radius-lg)] border border-[var(--yellow-light)] bg-[var(--yellow-tint)] p-4">
 	<div class="flex items-baseline gap-1">
 		<span class="font-[var(--font-display)] text-[28px] font-[300] text-[var(--color-text)] md:text-[40px]">
-			{score}
+			{score.toFixed(1)}
 		</span>
 		<span class="font-[var(--font-sans)] text-[13px] text-[var(--color-text-muted)]">/10</span>
 		<span class="ml-1 font-[var(--font-sans)] text-[12px] text-[var(--color-text-muted)]">Abwechslungs-Score</span>