DesktopNotifierSync: Add on_* properties for class-level events

[PhrozenByte]

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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 66.66667% with 8 lines in your changes missing coverage. Please review.

Project coverage is 84.59%. Comparing base (26c6af2) to head (90e7460).
Report is 14 commits behind head on main.

Files with missing lines	Patch %	Lines
src/desktop_notifier/sync.py	66.66%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #207      +/-   ##
==========================================
- Coverage   85.13%   84.59%   -0.55%     
==========================================
  Files          10       10              
  Lines         895      928      +33     
==========================================
+ Hits          762      785      +23     
- Misses        133      143      +10     

Flag	Coverage Δ
pytest	84.59% <66.66%> (-0.55%)	⬇️

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