fix: add explicit white color to default subtitle text

[nathannewyen]

Summary

• Added explicit color: Colors.white to the default subtitle text rendering in all control widgets
• Ensures subtitle text is always visible regardless of the app's theme configuration
• Subtitles are rendered on a semi-transparent black background (Color(0x96000000)), so dark text colors can be invisible

Changes

• lib/src/cupertino/cupertino_controls.dart - Added white color to subtitle TextStyle
• lib/src/material/material_controls.dart - Added white color to subtitle TextStyle
• lib/src/material/material_desktop_controls.dart - Added white color to subtitle TextStyle

Test plan

• [x] Tested with iOS/Cupertino controls - subtitles now visible
• [x] Tested with Material controls - subtitles remain visible
• [x] Verified fix applies only to default subtitle rendering (apps with custom subtitleBuilder are unaffected)

Addresses #914

🤖 Generated with Claude Code

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 43.84%. Comparing base (d228479) to head (c12d5bf). :warning: Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #934   +/-   ##
=======================================
  Coverage   43.84%   43.84%           
=======================================
  Files          21       21           
  Lines        1601     1601           
=======================================
  Hits          702      702           
  Misses        899      899           

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