Redesign incident viewer: details always visible, alerts/notes as sections

[clcollins]

Summary

• Details always visible at top (not a tab)
• Alerts and Notes as selectable sections below
• Up/Down switches active section, Tab/Shift+Tab cycles items within section
• 1-9 jumps to specific item in active section
• Section headers show ▸/▹ for active/inactive

Closes #235

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 84.00000% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.45%. Comparing base (0d0986a) to head (a3550cc).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/tui/msgHandlers.go	78.72%	2 Missing and 8 partials ⚠️
pkg/tui/views.go	88.23%	3 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #240      +/-   ##
==========================================
+ Coverage   67.73%   69.45%   +1.72%     
==========================================
  Files          26       26              
  Lines        3790     3781       -9     
==========================================
+ Hits         2567     2626      +59     
+ Misses       1088     1012      -76     
- Partials      135      143       +8     

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