refactor(ui): refactor glow effect drawing

[jamesarich]

Refactors the glow effect drawing logic within Modifier.drawWithCache. This change pre-calculates the radial gradient brush and applies the alpha dynamically, improving efficiency by avoiding brush recalculation on each animation step.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.57%. Comparing base (22240a4) to head (5ea2020).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
app/src/main/java/com/geeksville/mesh/ui/Main.kt	0.00%	15 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##            main   #3988      +/-   ##
========================================
- Coverage   0.57%   0.57%   -0.01%     
========================================
  Files        401     401              
  Lines      23692   23696       +4     
  Branches    3041    3042       +1     
========================================
  Hits         136     136              
- Misses     23535   23539       +4     
  Partials      21      21              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.