Fix week date range off-by-one in BST timezone

toISOString() returns UTC — in BST (UTC+1) dates computed from new Date()
land one day early. Replace with local date components via fmt()/localStr().
Affects Weekly, Monthly, Rolling12Weeks, Rolling12Months.

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

frontend/src/pages/Weekly.tsx

@@ -3,18 +3,22 @@ import { ChevronLeft, ChevronRight, Download } from 'lucide-react'
 import { getActuals, getNetSales, getBudgets, downloadExport } from '../api'
 import type { DeptActuals, WageBudget } from '../types'
 
+function localStr(d: Date): string {
+  return `${d.getFullYear()}-${String(d.getMonth() + 1).padStart(2, '0')}-${String(d.getDate()).padStart(2, '0')}`
+}
+
 function mondayOf(d: Date): string {
   const day = d.getDay()
   const r = new Date(d)
   r.setDate(d.getDate() + (day === 0 ? -6 : 1 - day))
   r.setHours(0, 0, 0, 0)
-  return r.toISOString().slice(0, 10)
+  return localStr(r)
 }
 
 function addDaysStr(dateStr: string, n: number): string {
   const d = new Date(dateStr + 'T00:00:00')
   d.setDate(d.getDate() + n)
-  return d.toISOString().slice(0, 10)
+  return localStr(d)
 }
 
 function daysInMonthFor(dateStr: string): number {
@@ -41,7 +45,7 @@ export default function Weekly() {
   const [fromStr, setFromStr] = useState<string>(() => mondayOf(new Date()))
 
   const toStr    = addDaysStr(fromStr, 6)
-  const todayStr = new Date().toISOString().slice(0, 10)
+  const todayStr = localStr(new Date())
 
   const [depts,       setDepts]       = useState<DeptActuals[]>([])
   const [netSales,    setNetSales]    = useState(0)