fix(web): add client-side email/password validation on login (#28562)

Firefox suppresses HTML5 validation tooltips for type=email inputs on
non-HTTPS pages, leaving users with no feedback when they submit an
invalid or empty email address. Add explicit client-side checks in
handleLogin before the API call so every browser shows a clear error
message in the Alert banner.

i18n/en.json

@@ -1027,6 +1027,7 @@
   "editor_rotate_left": "Rotate 90° counterclockwise",
   "editor_rotate_right": "Rotate 90° clockwise",
   "email": "Email",
+  "email_required": "Email is required",
   "email_notifications": "Email notifications",
   "empty_folder": "This folder is empty",
   "empty_trash": "Empty trash",

web/src/routes/auth/login/+page.svelte

@@ -78,6 +78,15 @@
   });
 
   const handleLogin = async () => {
+    if (!email || !email.includes('@')) {
+      errorMessage = email ? $t('errors.incorrect_email_or_password') : $t('email_required');
+      return;
+    }
+    if (!password) {
+      errorMessage = $t('password_required');
+      return;
+    }
+
     try {
       errorMessage = '';
       loading = true;