fix: Force unique usernames

Force unique usernames by using the username plugin for BetterAuth

src/lib/server/db/auth.ts

@@ -1,4 +1,5 @@
 import { betterAuth } from 'better-auth';
+import { username } from 'better-auth/plugins';
 import Database from 'better-sqlite3';
 
 export const auth = betterAuth({
@@ -7,4 +8,5 @@ export const auth = betterAuth({
     enabled: true,
     autoSignIn: true,
   },
+  plugins: [username()],
 });

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

@@ -37,6 +37,7 @@ export const actions = {
           name,
           email,
           password,
+          username: name,
         },
         asResponse: true,
       });