fix registration availability checks and signup endpoint

- correct api paths for username/email availability checks
- use singleton userAdapter instead of creating new instance
- use getApiBaseUrl() for signup to include /api/v1 prefix

src/lib/api/adapters/user.adapter.ts

@@ -197,7 +197,7 @@ export class UserAdapter extends BaseAdapter {
    * Check username availability
    */
   async checkUsernameAvailability(username: string): Promise<{ available: boolean }> {
-    return this.request<{ available: boolean }>(`/users/check-username`, {
+    return this.request<{ available: boolean }>(`/check/username`, {
       method: 'POST',
       body: JSON.stringify({ username })
     })
@@ -207,7 +207,7 @@ export class UserAdapter extends BaseAdapter {
    * Check email availability
    */
   async checkEmailAvailability(email: string): Promise<{ available: boolean }> {
-    return this.request<{ available: boolean }>(`/users/check-email`, {
+    return this.request<{ available: boolean }>(`/check/email`, {
       method: 'POST',
       body: JSON.stringify({ email })
     })

src/routes/auth/register/+page.svelte

@@ -3,7 +3,7 @@
 	import AuthCard from '$lib/components/auth/AuthCard.svelte'
 	import Input from '$lib/components/ui/Input.svelte'
 	import Button from '$lib/components/ui/Button.svelte'
-	import { UserAdapter } from '$lib/api/adapters/user.adapter'
+	import { userAdapter } from '$lib/api/adapters/user.adapter'
 	import * as m from '$lib/paraglide/messages'
 
 	interface Props {
@@ -38,8 +38,6 @@
 	let usernameTimer: ReturnType<typeof setTimeout>
 	let emailTimer: ReturnType<typeof setTimeout>
 
-	const userAdapter = new UserAdapter()
-
 	// Username validation regex
 	const usernameRegex = /^[a-zA-Z0-9_-]+$/
 	const emailRegex = /^[^\s@]+@[^\s@]+\.[^\s@]+$/

src/routes/auth/signup/+server.ts

@@ -2,7 +2,7 @@ import type { RequestHandler } from '@sveltejs/kit'
 import { json } from '@sveltejs/kit'
 import { dev } from '$app/environment'
 import { z } from 'zod'
-import { PUBLIC_SIERO_API_URL } from '$env/static/public'
+import { getApiBaseUrl } from '$lib/api/adapters/config'
 import { passwordGrantLogin } from '$lib/auth/oauth'
 import { userAdapter } from '$lib/api/adapters/user.adapter'
 import { buildCookies } from '$lib/auth/map'
@@ -40,7 +40,7 @@ export const POST: RequestHandler = async ({ request, cookies, fetch }) => {
 
 	try {
 		// 1. Create user account via API
-		const signupRes = await fetch(`${PUBLIC_SIERO_API_URL}/users`, {
+		const signupRes = await fetch(`${getApiBaseUrl()}/users`, {
 			method: 'POST',
 			headers: { 'Content-Type': 'application/json' },
 			body: JSON.stringify({