Add Prettier for TypeScript formatting

- Install prettier
- Configure .prettierrc.json and .prettierignore
- Add npm scripts: format, format:check
- Add Makefile target: format-frontend
- Format all frontend files

frontend/app/utils/date.ts

@@ -17,8 +17,8 @@ export function formatDate(d: Date): string {
  */
 export function formatTime(isoString: string): string {
   const d = new Date(isoString);
-  return d.toLocaleTimeString("en-US", { 
-    hour: "2-digit", 
+  return d.toLocaleTimeString("en-US", {
+    hour: "2-digit",
     minute: "2-digit",
     hour12: false,
   });
@@ -81,4 +81,3 @@ export function isWeekend(date: Date): boolean {
   const day = date.getDay();
   return day === 0 || day === 6;
 }
-