Add system clipboard integration behind clipboard feature

[ryanoneill]

Summary

• Add arboard = { version = "3", optional = true } dependency and clipboard feature flag
• Add clipboard to the full feature array
• Add system_clipboard_set() and system_clipboard_get() helpers to InputField and TextArea (behind #[cfg(feature = "clipboard")])
• Copy/cut now writes to both internal and system clipboard
• Paste prefers system clipboard, falls back to internal clipboard
• Errors silently ignored for headless environments (CI, SSH, containers)
• Update clipboard-related tests with #[cfg] gates for feature-dependent behavior

Test plan

• cargo test — all tests pass
• cargo clippy -- -D warnings — clean
• cargo check --no-default-features — compiles (clipboard not included)
• cargo test --doc — pass
• cargo build --examples — compiles

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 59.09091% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.56%. Comparing base (41832db) to head (b407eea).

Files with missing lines	Patch %	Lines
src/component/text_area/mod.rs	36.36%	7 Missing ⚠️
src/component/input_field/mod.rs	81.81%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #110      +/-   ##
==========================================
- Coverage   92.65%   92.56%   -0.09%     
==========================================
  Files          99       99              
  Lines        9491     9513      +22     
==========================================
+ Hits         8794     8806      +12     
- Misses        697      707      +10     

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