Fix login: use URLSearchParams instead of FormData

FastAPI OAuth2 expects application/x-www-form-urlencoded format.
FormData was sending multipart/form-data causing validation error.

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Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

frontend/src/api/auth.js

@@ -2,11 +2,11 @@ import client from './client'
 
 export default {
   async login(email, password) {
-    const formData = new FormData()
-    formData.append('username', email)
-    formData.append('password', password)
+    const params = new URLSearchParams()
+    params.append('username', email)
+    params.append('password', password)
 
-    const { data } = await client.post('/auth/login', formData, {
+    const { data } = await client.post('/auth/login', params, {
       headers: {
         'Content-Type': 'application/x-www-form-urlencoded'
       }