refactor: centralize validation, error handling, and formatting logic

Extract shared auth logic and validation rules to shared/auth.ts
Introduce utility functions for HTTP errors and user input parsing
Standardize error messages and date formatting across the app

server/utils/auth.ts

@@ -1,14 +1,10 @@
 import type { H3Event } from 'h3'
 
-import type { UserRole } from '~~/shared/auth'
+import { normalizeUsername, type UserRole } from '~~/shared/auth'
 
 import { createUserSession, destroyUserSession, getUserSession } from './session'
 import { getUserById, updateLastLogin, type UserAuthRecord } from './user-repository'
 
-export function normalizeUsername(value: string) {
-  return value.trim().toLowerCase()
-}
-
 export async function getAuthContext(event: H3Event): Promise<{
   session: Awaited<ReturnType<typeof getUserSession>>
   user: UserAuthRecord