Fix QA findings: firstDay reactivity, state revert, helper extraction

- W1: Add key prop to FullCalendar so firstDay change triggers remount
- W2: Revert firstDayOfWeek toggle state on API failure
- S1: Extract _rule_int helper in recurrence service to reduce duplication

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

backend/app/services/recurrence.py

@@ -28,6 +28,12 @@ def _nth_weekday_of_month(year: int, month: int, weekday: int, week: int) -> Opt
     return target
 
 
+def _rule_int(rule: dict, key: str, default: int) -> int:
+    """Get an int from the rule dict, falling back to default if missing or None."""
+    val = rule.get(key)
+    return int(val) if val is not None else default
+
+
 def generate_occurrences(
     parent: CalendarEvent,
     horizon_days: int = 365,
@@ -90,7 +96,7 @@ def generate_occurrences(
     occurrences.append(_make_child(parent_start))
 
     if rule_type == "every_n_days":
-        interval: int = int(rule.get("interval") or 1)
+        interval: int = _rule_int(rule, "interval", 1)
         if interval < 1:
             interval = 1
         current = parent_start + timedelta(days=interval)
@@ -99,7 +105,7 @@ def generate_occurrences(
             current += timedelta(days=interval)
 
     elif rule_type == "weekly":
-        weekday: int = int(rule.get("weekday") if rule.get("weekday") is not None else parent_start.weekday())
+        weekday: int = _rule_int(rule, "weekday", parent_start.weekday())
         # Start from the next week after the parent
         days_ahead = (weekday - parent_start.weekday()) % 7
         if days_ahead == 0:
@@ -110,8 +116,8 @@ def generate_occurrences(
             current += timedelta(weeks=1)
 
     elif rule_type == "monthly_nth_weekday":
-        week: int = int(rule.get("week") or 1)
+        week: int = _rule_int(rule, "week", 1)
-        weekday = int(rule.get("weekday") if rule.get("weekday") is not None else parent_start.weekday())
+        weekday = _rule_int(rule, "weekday", parent_start.weekday())
         # Advance month by month
         year, month = parent_start.year, parent_start.month
         # Move to the next month from the parent
@@ -144,7 +150,7 @@ def generate_occurrences(
                 break
 
     elif rule_type == "monthly_date":
-        day: int = int(rule.get("day") or parent_start.day)
+        day: int = _rule_int(rule, "day", parent_start.day)
         year, month = parent_start.year, parent_start.month
         month += 1
         if month > 12:

frontend/src/components/calendar/CalendarPage.tsx

@@ -322,6 +322,7 @@ export default function CalendarPage() {
         <div className="flex-1 overflow-y-auto">
           <div className="h-full">
             <FullCalendar
+              key={`fc-${settings?.first_day_of_week ?? 0}`}
               ref={calendarRef}
               plugins={[dayGridPlugin, timeGridPlugin, interactionPlugin]}
               initialView="dayGridMonth"

frontend/src/components/settings/SettingsPage.tsx

@@ -139,12 +139,14 @@ export default function SettingsPage() {
   };
 
   const handleFirstDayChange = async (value: number) => {
+    const previous = firstDayOfWeek;
     setFirstDayOfWeek(value);
     try {
       await updateSettings({ first_day_of_week: value });
       queryClient.invalidateQueries({ queryKey: ['calendar-events'] });
       toast.success(value === 0 ? 'Week starts on Sunday' : 'Week starts on Monday');
     } catch {
+      setFirstDayOfWeek(previous);
       toast.error('Failed to update first day of week');
     }
   };