[WIP] Add flashing taskbar icon for message notifications

[Copilot]

Plan: Add Taskbar Icon Flashing on Message Receipt

• Understand how DialogBase implements window urgency hint
• Add urgency hint to MainWindow when NEW_MESSAGE event is received
• Add focus event handler to clear urgency hint when MainWindow gains focus
• Test the implementation
• Run code review and security checks

Original prompt

---

This section details on the original issue you should resolve

<issue_title>消息闪动</issue_title>
<issue_description>大佬，可否让接收到消息后任务栏信使图标闪动提醒呢？谢谢</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes 消息闪动 #709

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.

[Copilot]

Copilot wasn't able to review any files in this pull request.

[github-actions]

Test Results

69 tests  ±0   69 ✅ ±0   3s ⏱️ ±0s
32 suites ±0    0 💤 ±0 
 1 files   ±0    0 ❌ ±0 

Results for commit d21ab51. ± Comparison against base commit e5ebeed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.00%. Comparing base (e5ebeed) to head (d21ab51).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #710      +/-   ##
==========================================
- Coverage   52.01%   52.00%   -0.02%     
==========================================
  Files          64       64              
  Lines        8599     8599              
==========================================
- Hits         4473     4472       -1     
- Misses       4126     4127       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.