Add client-side validation for taken username in sign-up form (#24546)

app/javascript/mastodon/locales/defaultMessages.json

@@ -4358,5 +4358,22 @@
       }
     ],
     "path": "app/javascript/mastodon/features/video/index.json"
+  },
+  {
+    "descriptors": [
+      {
+        "defaultMessage": "That username is taken. Try another",
+        "id": "username.taken"
+      },
+      {
+        "defaultMessage": "Password confirmation exceeds the maximum password length",
+        "id": "password_confirmation.exceeds_maxlength"
+      },
+      {
+        "defaultMessage": "Password confirmation does not match",
+        "id": "password_confirmation.mismatching"
+      }
+    ],
+    "path": "app/javascript/packs/public.json"
   }
 ]

app/javascript/mastodon/locales/en.json

@@ -443,6 +443,8 @@
   "notifications_permission_banner.enable": "Enable desktop notifications",
   "notifications_permission_banner.how_to_control": "To receive notifications when Mastodon isn't open, enable desktop notifications. You can control precisely which types of interactions generate desktop notifications through the {icon} button above once they're enabled.",
   "notifications_permission_banner.title": "Never miss a thing",
+  "password_confirmation.exceeds_maxlength": "Password confirmation exceeds the maximum password length",
+  "password_confirmation.mismatching": "Password confirmation does not match",
   "picture_in_picture.restore": "Put it back",
   "poll.closed": "Closed",
   "poll.refresh": "Refresh",
@@ -651,6 +653,7 @@
   "upload_modal.preview_label": "Preview ({ratio})",
   "upload_progress.label": "Uploading...",
   "upload_progress.processing": "Processing…",
+  "username.taken": "That username is taken. Try another",
   "video.close": "Close video",
   "video.download": "Download file",
   "video.exit_fullscreen": "Exit full screen",