fix(ui): widen navigation rail for macOS traffic lights

[monkeyWie]

The 68px primary navigation rail leaves too little horizontal space for newer macOS traffic lights. Increase the shared rail width to 78px across platforms and update the desktop layout specification.

Reduce extension cards' minimum width to 280px so the 1024px reference window keeps three columns. Let browser-extension button labels shrink in settings to avoid overflow at the 720px desktop boundary. Update the existing layout tests for these widths.

Validation:

• 10 targeted responsive widget tests passed, including extension-card action spacing and desktop/mobile navigation.
• Static analysis of the three changed Dart implementation files passed.
• git diff --check passed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 70.73%. Comparing base (c661979) to head (26d54e2).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1582      +/-   ##
==========================================
- Coverage   70.84%   70.73%   -0.12%     
==========================================
  Files          89       89              
  Lines       14442    14442              
==========================================
- Hits        10231    10215      -16     
- Misses       3080     3096      +16     
  Partials     1131     1131              

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