fix(planner): address all PR review blockers

- Fix logic bug `{#if !isPlanner === false}` - view/cook buttons now visible for all roles, swap only for planner
- Convert Tauschen from dead button to link with suggestions href
- Add week.ts unit tests (23 tests covering getWeekStart Sunday edge case, prevWeek/nextWeek, weekDays, isToday, formatWeekRange)
- Fix isToday to use UTC consistently (.toISOString().slice(0,10)) instead of local date
- Add server-side role guard to createPlan action (403 for members)
- Add weekStart format validation in createPlan action
- Add isSelected prop to DayMealCard with green treatment
- Make variety banner sticky on mobile (always visible per spec)
- Add day name abbreviation above date badge in desktop column headers
- Remove placeholder Navigation text from desktop sidebar
- Add aria-label to desktop empty tile buttons
- Add variety score partial failure test, multiple overlaps test, WeekStrip today+selected test

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

frontend/src/lib/planner/DayMealCard.svelte

@@ -15,10 +15,12 @@
 	let {
 		slot,
 		isToday = false,
+		isSelected = false,
 		readonly = false
 	}: {
 		slot: Slot;
 		isToday?: boolean;
+		isSelected?: boolean;
 		readonly?: boolean;
 	} = $props();
 
@@ -30,13 +32,21 @@
 			.filter(Boolean)
 			.join(' · ')
 	);
+
+	let borderClass = $derived(
+		isToday
+			? 'border-[var(--yellow)] bg-[var(--yellow-tint)]'
+			: isSelected
+				? 'border-[var(--green)] bg-[var(--green-tint)]'
+				: 'border-[var(--color-border)] bg-[var(--color-surface)]'
+	);
 </script>
 
 <div
 	data-testid="day-meal-card"
 	data-today={isToday}
-	class="rounded-[var(--radius-lg)] border-2 p-4 transition-colors
-		{isToday ? 'border-[var(--yellow)] bg-[var(--yellow-tint)]' : 'border-[var(--color-border)] bg-[var(--color-surface)]'}"
+	data-selected={isSelected}
+	class="rounded-[var(--radius-lg)] border-2 p-4 transition-colors {borderClass}"
 >
 	{#if slot.recipe}
 		<h3 class="font-[var(--font-display)] text-[20px] font-[300] leading-tight text-[var(--color-text)]">
@@ -54,19 +64,19 @@
 				>
 					Jetzt kochen
 				</a>
-				<button
-					type="button"
+				<a
+					href="/planner/suggestions?day={slot.slotDate}"
 					class="rounded-[var(--radius-md)] border border-[var(--color-border)] px-3 py-2 text-[13px] font-medium tracking-[0.04em] font-[var(--font-sans)] text-[var(--color-text)]"
 				>
 					Tauschen
-				</button>
+				</a>
 			</div>
 		{/if}
 	{:else}
 		<p class="font-[var(--font-sans)] text-[14px] text-[var(--color-text-muted)]">Kein Gericht geplant</p>
 		{#if !readonly}
 			<a
-				href="/planner/suggestions"
+				href="/planner/suggestions?day={slot.slotDate}"
 				class="mt-2 inline-block rounded-[var(--radius-md)] border border-dashed border-[var(--color-border)] px-3 py-2 text-[13px] font-medium tracking-[0.04em] font-[var(--font-sans)] text-[var(--color-text-muted)]"
 			>
 				+ Gericht hinzufügen