fix: improve error logging for external command notifications

[linw1995]

Closes #140

Summary

This PR improves error handling and logging for external command notifications, making it easier to troubleshoot configuration issues.

Changes

• Added warn level logs when external command execution fails at any stage (spawn, write to stdin, wait, exit status)
• Added structured logging with relevant context (executable path, args, exit status)
• Added normalize_executable_path function to expand tilde (~) in executable paths using HOME environment variable
• Improved error handling with explicit match statements instead of silent failures
• Added unit test for tilde expansion in executable paths

[codecov-commenter]

Codecov Report

❌ Patch coverage is 42.22222% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 48.85%. Comparing base (a46f02c) to head (1f0dbf6).
⚠️ Report is 5 commits behind head on dev.

Files with missing lines	Patch %	Lines
crates/coco-tui/src/notifications.rs	42.22%	26 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #142      +/-   ##
==========================================
- Coverage   50.22%   48.85%   -1.38%     
==========================================
  Files          93       95       +2     
  Lines       25033    26112    +1079     
==========================================
+ Hits        12574    12756     +182     
- Misses      12459    13356     +897     

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