Fix: Show error in UI when rule saving fails

[gsiri-jack]

This PR enhances the user experience by adding proper UI feedback when saving rules fails due to API/network issues.

• Added errorMessage reactive ref
• Displayed error in <App.vue> using a styled banner
• Included graceful fallback in watch(rules) to notify the user via mw.notify

This is a follow-up to #52, which was closed due to a syntax issue.

Fixes ProfessionalWiki/Rules#43

[malberts]

Please attach a screenshot of this running in a wiki.

Also, this does not follow our coding standards and there is an obvious formatting issue here:

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.62%. Comparing base (241a552) to head (7b9fdfc).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master      #54      +/-   ##
============================================
+ Coverage     60.47%   61.62%   +1.14%     
- Complexity       63       64       +1     
============================================
  Files            11       11              
  Lines           167      172       +5     
============================================
+ Hits            101      106       +5     
  Misses           66       66              

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