feat(settings): first day of week (plan Tier 3)

- New per-user 'week_start' setting (0=Sun default / 1=Mon), registered in
  USER_SETTING_KEYS + USER_SETTING_DEFAULTS.
- CalendarPage reads it and offsets the month grid: leading-blank count
  (getDay()-weekStart+7)%7 and a rotated weekday header. Purely presentational.
- Settings: Sun/Mon Select in the new "Regional" section. Also aligns the
  grace_period_days client default (3→5) with the server seed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

client/pages/CalendarPage.tsx

@@ -18,6 +18,7 @@ import { api } from '@/api';
 import { cn, fmtDate, todayStr, errMessage } from '@/lib/utils';
 import { formatUSD, asDollars } from '@/lib/money';
 import { isPaidStatus } from '@/lib/trackerUtils';
+import { useSettings } from '@/hooks/useQueries';
 import { Button } from '@/components/ui/button';
 import { Badge } from '@/components/ui/badge';
 import { Card, CardContent, CardHeader, CardTitle, CardDescription } from '@/components/ui/card';
@@ -395,13 +396,17 @@ function DayIndicators({ day, moneyMarker }: { day: CalDay; moneyMarker: MoneyMa
   );
 }
 
-function CalendarGrid({ data, selectedDate, onSelectDay, moneyMarkers }: {
+function CalendarGrid({ data, selectedDate, onSelectDay, moneyMarkers, weekStart = 0 }: {
   data: CalendarData;
   selectedDate?: string;
   onSelectDay: (day: CalDay) => void;
   moneyMarkers?: Record<string, MoneyMarker>;
+  weekStart?: number;
 }) {
-  const firstWeekday = new Date(data.year, data.month - 1, 1).getDay();
+  // Offset the leading blanks + rotate the header so the week can start on
+  // Sunday (0) or Monday (1) per the user's setting.
+  const firstWeekday = (new Date(data.year, data.month - 1, 1).getDay() - weekStart + 7) % 7;
+  const orderedWeekdays = [...WEEKDAYS.slice(weekStart), ...WEEKDAYS.slice(0, weekStart)];
   const cells: ({ type: 'blank'; key: string } | { type: 'day'; key: string; day: CalDay })[] = [
     ...Array.from({ length: firstWeekday }, (_, index) => ({ type: 'blank' as const, key: `blank-${index}` })),
     ...data.days.map(day => ({ type: 'day' as const, key: day.date, day })),
@@ -411,7 +416,7 @@ function CalendarGrid({ data, selectedDate, onSelectDay, moneyMarkers }: {
   return (
     <Card className="overflow-hidden border-border/80 bg-card/95">
       <div className="grid grid-cols-7 border-b border-border/70 bg-muted/45">
-        {WEEKDAYS.map(day => (
+        {orderedWeekdays.map(day => (
           <div key={day} className="px-1 py-2 text-center text-[11px] font-semibold uppercase tracking-wide text-foreground/70 sm:text-xs">
             {day}
           </div>
@@ -911,6 +916,8 @@ export default function CalendarPage() {
   })();
   const [year, setYear] = useState(initial.year);
   const [month, setMonth] = useState(initial.month);
+  const { data: settingsData } = useSettings();
+  const weekStart = Number((settingsData as Record<string, unknown> | undefined)?.week_start) === 1 ? 1 : 0;
   const [data, setData] = useState<CalendarData | null>(null);
   const [summaryData, setSummaryData] = useState<SummaryDataShape | null>(null);
   const [snowballProjection, setSnowballProjection] = useState<SnowballProjection | null>(null);
@@ -1063,6 +1070,7 @@ export default function CalendarPage() {
             <>
               <CalendarGrid
                 data={data}
+                weekStart={weekStart}
                 selectedDate={selectedDay?.date}
                 moneyMarkers={moneyMarkers}
                 onSelectDay={day => {

client/pages/SettingsPage.tsx

@@ -314,6 +314,7 @@ function LinkImportToggle() {
 interface SettingsState {
   currency: string;
   date_format: string;
+  week_start: string;
   grace_period_days: number | string;
   drift_threshold_pct: string;
   tracker_show_bank_projection_banner: string;
@@ -332,7 +333,8 @@ export default function SettingsPage() {
   const DEFAULTS: SettingsState = {
     currency:              'USD',
     date_format:           'MM/DD/YYYY',
-    grace_period_days:     3,
+    week_start:            '0',
+    grace_period_days:     5,
     drift_threshold_pct:   '5',
     tracker_show_bank_projection_banner: 'true',
     tracker_bank_projection_banner_snoozed_until: '',
@@ -363,6 +365,7 @@ export default function SettingsPage() {
   const buildPayload = (s: SettingsState) => ({
     currency: s.currency,
     date_format: s.date_format,
+    week_start: s.week_start,
     grace_period_days: s.grace_period_days,
     drift_threshold_pct: s.drift_threshold_pct,
     tracker_show_bank_projection_banner: s.tracker_show_bank_projection_banner,
@@ -476,6 +479,18 @@ export default function SettingsPage() {
             </SelectContent>
           </Select>
         </SettingRow>
+
+        <SettingRow label="First day of week" description="The starting column of the calendar month grid.">
+          <Select value={settings.week_start} onValueChange={(v) => set('week_start', v)}>
+            <SelectTrigger className="w-48">
+              <SelectValue />
+            </SelectTrigger>
+            <SelectContent>
+              <SelectItem value="0">Sunday</SelectItem>
+              <SelectItem value="1">Monday</SelectItem>
+            </SelectContent>
+          </Select>
+        </SettingRow>
       </SectionCard>
 
       {/* Tracker Layout */}

services/userSettings.js

@@ -5,6 +5,7 @@ const { getDb, getSetting } = require('../db/database');
 const USER_SETTING_KEYS = [
   'currency',
   'date_format',
+  'week_start',
   'grace_period_days',
   'notify_days_before',
   'drift_threshold_pct',
@@ -29,6 +30,7 @@ const USER_SETTING_KEYS = [
 ];
 
 const USER_SETTING_DEFAULTS = {
+  week_start: '0',
   search_bars_collapsed: 'false',
   bank_auto_categorize_merchants: 'true',
   tracker_show_bank_projection_banner: 'true',