chore(pubspec): remove unused Flutter-root dependencies

[leehack]

Summary

• Remove unused Flutter-only constraints and dependencies from root pubspec.yaml to keep the core package pure Dart.
• Keep Flutter-specific dependencies scoped to example apps where they are actually used.
• Add an Unreleased changelog entry describing the metadata cleanup.

Validation

• dart analyze
• dart test

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.26%. Comparing base (fe8dbde) to head (f7c8bf7).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #67   +/-   ##
=======================================
  Coverage   77.26%   77.26%           
=======================================
  Files          66       66           
  Lines        8046     8046           
=======================================
  Hits         6217     6217           
  Misses       1829     1829           

Flag	Coverage Δ
unittests	77.26% <ø> (ø)

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