feat(planner): implement C1 weekly planner home screen (#26)

Three-breakpoint layout (mobile/tablet/desktop) with VarietyScoreCard,
WeekStrip, DayMealCard components. Server loads week plan and variety
score via API; read-only role behavior derived from benutzer.rolle.

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

frontend/src/lib/planner/week.ts

@@ -0,0 +1,88 @@
+/**
+ * Returns the ISO Monday (YYYY-MM-DD) for the week containing `date`.
+ */
+export function getWeekStart(date: Date): string {
+	const d = new Date(date);
+	const day = d.getUTCDay(); // 0=Sun, 1=Mon, …
+	const diff = day === 0 ? -6 : 1 - day; // shift to Monday
+	d.setUTCDate(d.getUTCDate() + diff);
+	return d.toISOString().slice(0, 10);
+}
+
+/**
+ * Returns the Monday of the previous week relative to `weekStart`.
+ */
+export function prevWeek(weekStart: string): string {
+	const d = new Date(weekStart + 'T00:00:00Z');
+	d.setUTCDate(d.getUTCDate() - 7);
+	return d.toISOString().slice(0, 10);
+}
+
+/**
+ * Returns the Monday of the next week relative to `weekStart`.
+ */
+export function nextWeek(weekStart: string): string {
+	const d = new Date(weekStart + 'T00:00:00Z');
+	d.setUTCDate(d.getUTCDate() + 7);
+	return d.toISOString().slice(0, 10);
+}
+
+/**
+ * Formats a date string (YYYY-MM-DD) as a localized day abbreviation.
+ */
+export function formatDayAbbr(dateStr: string, length: 'narrow' | 'short' = 'narrow'): string {
+	const d = new Date(dateStr + 'T00:00:00Z');
+	return d.toLocaleDateString('de-DE', { weekday: length, timeZone: 'UTC' });
+}
+
+/**
+ * Returns an array of 7 date strings for the week starting on `weekStart`.
+ */
+export function weekDays(weekStart: string): string[] {
+	const days: string[] = [];
+	for (let i = 0; i < 7; i++) {
+		const d = new Date(weekStart + 'T00:00:00Z');
+		d.setUTCDate(d.getUTCDate() + i);
+		days.push(d.toISOString().slice(0, 10));
+	}
+	return days;
+}
+
+/**
+ * Formats a date string as "Mo, 30.03." style label.
+ */
+export function formatDayLabel(dateStr: string): string {
+	const d = new Date(dateStr + 'T00:00:00Z');
+	const day = d.toLocaleDateString('de-DE', { weekday: 'short', timeZone: 'UTC' });
+	const date = d.toLocaleDateString('de-DE', { day: '2-digit', month: '2-digit', timeZone: 'UTC' });
+	return `${day}, ${date}`;
+}
+
+/**
+ * Formats a date string as "30. März" style label.
+ */
+export function formatDayFull(dateStr: string): string {
+	const d = new Date(dateStr + 'T00:00:00Z');
+	return d.toLocaleDateString('de-DE', { day: 'numeric', month: 'long', timeZone: 'UTC' });
+}
+
+/**
+ * Returns true if dateStr is today (local time).
+ */
+export function isToday(dateStr: string): boolean {
+	const today = new Date();
+	const todayStr = `${today.getFullYear()}-${String(today.getMonth() + 1).padStart(2, '0')}-${String(today.getDate()).padStart(2, '0')}`;
+	return dateStr === todayStr;
+}
+
+/**
+ * Formats a week range: "30. Mär – 5. Apr 2026".
+ */
+export function formatWeekRange(weekStart: string): string {
+	const start = new Date(weekStart + 'T00:00:00Z');
+	const end = new Date(weekStart + 'T00:00:00Z');
+	end.setUTCDate(end.getUTCDate() + 6);
+	const startStr = start.toLocaleDateString('de-DE', { day: 'numeric', month: 'short', timeZone: 'UTC' });
+	const endStr = end.toLocaleDateString('de-DE', { day: 'numeric', month: 'short', year: 'numeric', timeZone: 'UTC' });
+	return `${startStr} – ${endStr}`;
+}