Fix plugin permission, PAGX file association, and viewer stability issues.

[CodeJhF]

Summary

• Fix H264EncoderTools installing to root-owned directory on macOS by separating User/Admin permission paths
• Fix PAGX files not loading when opened via file association (async signal-based file open flow)
• Fix crash caused by nested event loop in PAGNetworkFetcher (replaced with async fetch)
• Fix PAGX initial render showing blank content and multiple update checks on multi-window
• Improve ContentView lifecycle management with proper drawable release and timer safety
• Add APPDATA empty check on Windows and fix cross-thread signal connection in update model

Test plan

• Open .pagx file via file association on macOS and Windows, verify it loads correctly
• Install/uninstall H264EncoderTools plugin, verify no admin prompt on macOS
• Open multiple viewer windows, verify update check only runs once
• Switch between PAG and PAGX files in the same window, verify no crash
• Verify plugin install works correctly when APPDATA is set (Windows)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.49%. Comparing base (b3e2957) to head (629158f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3418      +/-   ##
==========================================
+ Coverage   78.41%   78.49%   +0.07%     
==========================================
  Files         532      532              
  Lines       41448    41448              
  Branches    12486    12486              
==========================================
+ Hits        32502    32535      +33     
+ Misses       6387     6380       -7     
+ Partials     2559     2533      -26     

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