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/week.ts

@@ -67,11 +67,11 @@ export function formatDayFull(dateStr: string): string {
 }
 
 /**
- * Returns true if dateStr is today (local time).
+ * Returns true if dateStr is today (UTC date).
+ * Uses UTC consistently with all other date functions in this module.
  */
 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')}`;
+	const todayStr = new Date().toISOString().slice(0, 10);
 	return dateStr === todayStr;
 }