Add French holidays to the timesheet, including Easter Monday

Implements logic to calculate and display French national holidays, such as Easter Monday, within the timesheet view.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: 55837015-10e9-4be9-b857-7f5e6be73772
Replit-Commit-Checkpoint-Type: full_checkpoint
Replit-Commit-Event-Id: 981f81d9-47f7-41fe-b3b4-19b5c1d2aa5d
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/1cc377db-7ea0-49f2-97ce-c3e87e0228cc/55837015-10e9-4be9-b857-7f5e6be73772/KSYTI3T
Replit-Helium-Checkpoint-Created: true

artifacts/cra-app/src/pages/timesheet-detail.tsx

@@ -38,6 +38,51 @@ import { useAdminUnlocked } from "@/lib/admin-mode";
 import { getDaysInMonth, isWeekend, format } from "date-fns";
 import { fr } from "date-fns/locale";
 
+function addDays(date: Date, days: number) {
+  const next = new Date(date);
+  next.setDate(next.getDate() + days);
+  return next;
+}
+
+function getEasterSunday(year: number) {
+  const a = year % 19;
+  const b = Math.floor(year / 100);
+  const c = year % 100;
+  const d = Math.floor(b / 4);
+  const e = b % 4;
+  const f = Math.floor((b + 8) / 25);
+  const g = Math.floor((b - f + 1) / 3);
+  const h = (19 * a + b - d - g + 15) % 30;
+  const i = Math.floor(c / 4);
+  const k = c % 4;
+  const l = (32 + 2 * e + 2 * i - h - k) % 7;
+  const m = Math.floor((a + 11 * h + 22 * l) / 451);
+  const month = Math.floor((h + l - 7 * m + 114) / 31);
+  const day = ((h + l - 7 * m + 114) % 31) + 1;
+  return new Date(year, month - 1, day);
+}
+
+function getFrenchHolidayName(date: Date) {
+  const year = date.getFullYear();
+  const dateStr = format(date, "yyyy-MM-dd");
+  const easterSunday = getEasterSunday(year);
+  const holidays: Record<string, string> = {
+    [`${year}-01-01`]: "Jour de l'an",
+    [format(addDays(easterSunday, 1), "yyyy-MM-dd")]: "Lundi de Pâques",
+    [`${year}-05-01`]: "Fête du Travail",
+    [`${year}-05-08`]: "Victoire 1945",
+    [format(addDays(easterSunday, 39), "yyyy-MM-dd")]: "Ascension",
+    [format(addDays(easterSunday, 50), "yyyy-MM-dd")]: "Lundi de Pentecôte",
+    [`${year}-07-14`]: "Fête nationale",
+    [`${year}-08-15`]: "Assomption",
+    [`${year}-11-01`]: "Toussaint",
+    [`${year}-11-11`]: "Armistice 1918",
+    [`${year}-12-25`]: "Noël",
+  };
+
+  return holidays[dateStr] || null;
+}
+
 type LocalEntry = {
   timesheetLineId: number;
   date: string;
@@ -100,10 +145,13 @@ export default function TimesheetDetailPage() {
     const daysCount = getDaysInMonth(new Date(timesheet.year, timesheet.month - 1));
     const arr = Array.from({ length: daysCount }, (_, i) => {
       const date = new Date(timesheet.year, timesheet.month - 1, i + 1);
+      const holidayName = getFrenchHolidayName(date);
       return {
         dateStr: format(date, "yyyy-MM-dd"),
         dayNum: i + 1,
         isWeekendDay: isWeekend(date),
+        isHoliday: Boolean(holidayName),
+        holidayName,
         dayName: format(date, "EE", { locale: fr })
       };
     });
@@ -404,8 +452,9 @@ export default function TimesheetDetailPage() {
                     key={day.dateStr} 
                     className={cn(
                       "px-0 py-1 font-medium text-center border-b border-r",
-                      day.isWeekendDay ? "bg-muted/50" : ""
+                      day.isWeekendDay || day.isHoliday ? "bg-muted/50" : ""
                     )}
+                    title={day.holidayName || undefined}
                   >
                     <div className="flex flex-col items-center leading-tight">
                       <span className="text-[9px] uppercase opacity-60">{day.dayName.slice(0, 2)}</span>
@@ -466,11 +515,11 @@ export default function TimesheetDetailPage() {
                           key={day.dateStr}
                           className={cn(
                             "p-0 text-center border-r transition-colors select-none",
-                            day.isWeekendDay ? "bg-muted/30" : "",
+                            day.isWeekendDay || day.isHoliday ? "bg-muted/30" : "",
                             isEditable ? "cursor-pointer hover:bg-primary/10" : "",
                             val > 0 ? "bg-primary/5 text-primary font-semibold" : ""
                           )}
-                          title={desc || undefined}
+                          title={desc || day.holidayName || undefined}
                         >
                           {isEditable ? (
                             <Popover>