Add data-testid to toast, update selector and login test

- Add data-testid='toast' to toast element in ui-utils.js
- Update toast selector in ui_keywords.resource to use [data-testid='toast']
- Update login rate limit test to check for 'invalid' instead of 'lockout'
  (testing mode disables lockout, so test verifies proper error message)
- Remove completed task docs from tasks.md

src/server/web/static/js/shared/ui-utils.js

@@ -32,6 +32,7 @@ AniWorld.UI = (function() {
 
         const toast = document.createElement('div');
         toast.className = 'toast ' + type;
+        toast.setAttribute('data-testid', 'toast');
         toast.innerHTML = 
             '<div style="display: flex; justify-content: space-between; align-items: center;">' +
                 '<span>' + escapeHtml(message) + '</span>' +