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

tests/robot/resources/ui_keywords.resource

@@ -185,7 +185,7 @@ Element Text Should Contain
 Toast Should Contain
     [Arguments]    ${text}
     [Documentation]    Assert that a toast/notification contains the given text.
-    ${toast}=    Get Text    .toast, .notification
+    ${toast}=    Get Text    [data-testid="toast"]
     Should Contain    ${toast}    ${text}
 
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