fix: rename login field from 'email' to '_username'

Symfony FormLoginAuthenticator expects _username by default.
The field used name="email" which caused 400 Bad Request on submit.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

templates/security/login.html.twig

@@ -0,0 +1,24 @@
+{% extends 'base.html.twig' %}
+
+{% block title %}Login{% endblock %}
+
+{% block body %}
+<div style="max-width:400px;margin:4rem auto;font-family:sans-serif">
+    <h1>SuperSeller Login</h1>
+    {% if error %}
+        <div style="color:red;margin-bottom:1rem">{{ error.messageKey|trans(error.messageData, 'security') }}</div>
+    {% endif %}
+    <form method="post">
+        <div style="margin-bottom:1rem">
+            <label for="email">Email</label><br>
+            <input id="email" type="email" name="_username" value="{{ last_username }}" required autofocus style="width:100%;padding:.5rem">
+        </div>
+        <div style="margin-bottom:1rem">
+            <label for="password">Password</label><br>
+            <input id="password" type="password" name="password" required style="width:100%;padding:.5rem">
+        </div>
+        <input type="hidden" name="_csrf_token" value="{{ csrf_token('authenticate') }}">
+        <button type="submit" style="padding:.5rem 1rem">Login</button>
+    </form>
+</div>
+{% endblock %}