Fix week nav arrows — use local date string to avoid BST offset bug

toISOString() returns UTC, so midnight local in BST (UTC+1) lands on
the previous UTC day, making each arrow step 6 days instead of 7.

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

frontend/src/pages/WeeklyActual.tsx

@@ -31,19 +31,22 @@ const MONTH_NAMES = ['January','February','March','April','May','June','July','A
 
 // ── Week navigation ────────────────────────────────────────────────────────
 
+function localDateStr(d: Date): string {
+  return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, '0')}-${String(d.getDate()).padStart(2, '0')}`
+}
+
 function defaultWeekEnding(): string {
   const d = new Date()
   const day = d.getDay()
-  // Go back to the most recent Saturday (day=6→today, else go back)
   const daysBack = day === 6 ? 0 : day + 1
   d.setDate(d.getDate() - daysBack)
-  return d.toISOString().split('T')[0]
+  return localDateStr(d)
 }
 
 function shiftWeek(dateStr: string, weeks: number): string {
   const d = new Date(dateStr + 'T00:00:00')
   d.setDate(d.getDate() + weeks * 7)
-  return d.toISOString().split('T')[0]
+  return localDateStr(d)
 }
 
 // ── Chart colours ──────────────────────────────────────────────────────────