refactor(variety): fix \$derived.by pattern, remove dead import, use pure functions

- Change all \$derived(() => {...}) to \$derived.by(() => {...}) — values not functions
- Remove unused formatDayLabel import
- Delegate subScores to computeSubScores(), warnings to computeWarnings()
- Remove () call syntax from all template reactive references

Addresses Kai blockers: anti-pattern derived, dead import.
Addresses QA blocker: logic now exercised by unit tests in variety.ts.

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

frontend/src/routes/(app)/planner/variety/+page.svelte

@@ -3,7 +3,7 @@
 	import ScoreBreakdownList from '$lib/planner/ScoreBreakdownList.svelte';
 	import VarietyWarningCards from '$lib/planner/VarietyWarningCards.svelte';
 	import EffortBar from '$lib/planner/EffortBar.svelte';
-	import { formatDayLabel } from '$lib/planner/week';
+	import { computeSubScores, computeWarnings } from '$lib/planner/variety';
 
 	let { data } = $props();
 
@@ -14,7 +14,7 @@
 	let score = $derived(varietyScore?.score ?? 0);
 
 	// Derive effort distribution from week plan slots
-	let effortCounts = $derived(() => {
+	let effortCounts = $derived.by(() => {
 		const slots = weekPlan?.slots ?? [];
 		let easy = 0, medium = 0, hard = 0;
 		for (const slot of slots) {
@@ -28,65 +28,21 @@
 
 	// Derive sub-scores from API data
 	// TODO: replace with API-provided sub-scores once backend supports them.
-	let subScores = $derived(() => {
-		const proteinRepeats = (varietyScore?.tagRepeats ?? []).filter(
-			(t: any) => t.tagType === 'protein'
-		).length;
-		const ingredientOverlapCount = (varietyScore?.ingredientOverlaps ?? []).length;
-		const { easy, medium, hard } = effortCounts();
-		const total = easy + medium + hard;
-		// Effort balance: ideal is roughly equal split; penalise extreme distributions
-		const effortBalance =
-			total === 0
-				? 10
-				: Math.round(Math.max(0, 10 - Math.abs(easy - hard) * 1.5));
-
-		return {
-			proteinDiversity: Math.max(0, Math.round(10 - proteinRepeats * 2)),
-			ingredientOverlap: Math.max(0, Math.round(10 - ingredientOverlapCount * 1.5)),
-			effortBalance: Math.min(10, effortBalance)
-		};
-	});
+	let subScores = $derived.by(() => computeSubScores({
+		tagRepeats: varietyScore?.tagRepeats ?? [],
+		ingredientOverlaps: varietyScore?.ingredientOverlaps ?? [],
+		...effortCounts
+	}));
 
 	// Build warning list from API data
-	let warnings = $derived(() => {
-		const result: { title: string; explanation: string }[] = [];
-
-		// Protein repeats
-		for (const repeat of varietyScore?.tagRepeats ?? []) {
-			if ((repeat.days?.length ?? 0) > 1) {
-				const days = (repeat.days ?? []).map((d: string) => d).join(', ');
-				result.push({
-					title: `${repeat.tagName} mehrfach diese Woche`,
-					explanation: `${days} — erwäge einen Tausch für mehr Protein-Abwechslung.`
-				});
-			}
-		}
-
-		// Ingredient overlaps
-		for (const overlap of varietyScore?.ingredientOverlaps ?? []) {
-			if ((overlap.days?.length ?? 0) > 1) {
-				const days = (overlap.days ?? []).join(', ');
-				result.push({
-					title: `${overlap.ingredientName} in mehreren Gerichten`,
-					explanation: `${days} — sorge für Zutaten-Abwechslung.`
-				});
-			}
-		}
-
-		// Duplicate recipes in plan
-		for (const name of varietyScore?.duplicatesInPlan ?? []) {
-			result.push({
-				title: `${name} doppelt geplant`,
-				explanation: 'Dasselbe Rezept erscheint mehrfach — tausche eines aus.'
-			});
-		}
-
-		return result;
-	});
+	let warnings = $derived.by(() => computeWarnings({
+		tagRepeats: varietyScore?.tagRepeats ?? [],
+		ingredientOverlaps: varietyScore?.ingredientOverlaps ?? [],
+		duplicatesInPlan: varietyScore?.duplicatesInPlan ?? []
+	}));
 
 	// Protein grid: map protein tags to days of the week
-	let proteinByDay = $derived(() => {
+	let proteinByDay = $derived.by(() => {
 		const map: Record<string, string> = {};
 		for (const repeat of varietyScore?.tagRepeats ?? []) {
 			if (repeat.tagType === 'protein') {
@@ -147,16 +103,16 @@
 				<h2 class="mb-3 font-[var(--font-sans)] text-[12px] font-medium uppercase tracking-wide text-[var(--color-text-muted)]">
 					Bewertung im Detail
 				</h2>
-				<ScoreBreakdownList subScores={subScores()} />
+				<ScoreBreakdownList {subScores} />
 			</div>
 
 			<!-- Warnings -->
-			{#if warnings().length > 0}
+			{#if warnings.length > 0}
 				<div class="space-y-3">
 					<h2 class="font-[var(--font-sans)] text-[12px] font-medium uppercase tracking-wide text-[var(--color-text-muted)]">
 						Hinweise
 					</h2>
-					<VarietyWarningCards warnings={warnings()} />
+					<VarietyWarningCards {warnings} />
 				</div>
 			{/if}
 		{/if}
@@ -208,7 +164,7 @@
 							<h2 class="mb-3 font-[var(--font-sans)] text-[12px] font-medium uppercase tracking-wide text-[var(--color-text-muted)]">
 								Bewertung im Detail
 							</h2>
-							<ScoreBreakdownList subScores={subScores()} />
+							<ScoreBreakdownList {subScores} />
 						</div>
 					</div>
 
@@ -220,10 +176,10 @@
 								Protein-Verteilung
 							</h2>
 							<div class="grid grid-cols-7 gap-[6px]">
-								{#each weekDayAbbrs as abbr, i}
+								{#each weekDayAbbrs as abbr, i (weekDayKeys[i])}
 									{@const key = weekDayKeys[i]}
-									{@const protein = proteinByDay()[key]}
-									{@const isRepeated = protein && Object.values(proteinByDay()).filter((p) => p === protein).length > 1}
+									{@const protein = proteinByDay[key]}
+									{@const isRepeated = protein && Object.values(proteinByDay).filter((p) => p === protein).length > 1}
 									<div class="flex flex-col items-center gap-1">
 										<span class="font-[var(--font-sans)] text-[10px] text-[var(--color-text-muted)]">{abbr}</span>
 										<div
@@ -247,11 +203,11 @@
 							<h2 class="mb-3 font-[var(--font-sans)] text-[12px] font-medium uppercase tracking-wide text-[var(--color-text-muted)]">
 								Aufwandsverteilung
 							</h2>
-							{#if (effortCounts().easy + effortCounts().medium + effortCounts().hard) > 0}
+							{#if (effortCounts.easy + effortCounts.medium + effortCounts.hard) > 0}
 								<EffortBar
-									easy={effortCounts().easy}
-									medium={effortCounts().medium}
-									hard={effortCounts().hard}
+									easy={effortCounts.easy}
+									medium={effortCounts.medium}
+									hard={effortCounts.hard}
 								/>
 							{:else}
 								<p class="font-[var(--font-sans)] text-[12px] text-[var(--color-text-muted)]">
@@ -263,12 +219,12 @@
 				</div>
 
 				<!-- Bottom: warnings, full width -->
-				{#if warnings().length > 0}
+				{#if warnings.length > 0}
 					<div class="mt-8 space-y-3">
 						<h2 class="font-[var(--font-sans)] text-[12px] font-medium uppercase tracking-wide text-[var(--color-text-muted)]">
 							Hinweise
 						</h2>
-						<VarietyWarningCards warnings={warnings()} />
+						<VarietyWarningCards {warnings} />
 					</div>
 				{/if}
 			{/if}