Add Dracula, Solarized Dark, and Gruvbox Dark built-in themes

[ryanoneill]

Summary

• Add Dracula color palette constants (11 colors) and Theme::dracula() constructor
• Add Solarized Dark color palette constants (12 colors) and Theme::solarized_dark() constructor
• Add Gruvbox Dark color palette constants (11 colors) and Theme::gruvbox_dark() constructor
• Add field verification and color constant tests for each new theme, plus a distinctness test across all 5 themes
• Update themed_app example to cycle through all 5 themes

Test plan

• cargo test — all tests pass (29 theme tests including 7 new)
• cargo clippy -- -D warnings — clean
• cargo check --no-default-features — compiles
• cargo test --doc — 270 doc tests pass (including 3 new theme docs)
• cargo build --examples — compiles

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.65%. Comparing base (41832db) to head (0af2c2a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #108   +/-   ##
=======================================
  Coverage   92.65%   92.65%           
=======================================
  Files          99       99           
  Lines        9491     9494    +3     
=======================================
+ Hits         8794     8797    +3     
  Misses        697      697           

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