Fix clipboard heap corruption on Windows

[ryanoneill]

Summary

• Root cause: arboard::Clipboard::new() initializes COM on Windows. When input_field and text_area tests run in parallel threads, concurrent COM init/teardown corrupts the heap, causing STATUS_HEAP_CORRUPTION (0xc0000374).
• Fix: Extract clipboard access into a shared src/clipboard.rs module with a thread_local! cached arboard::Clipboard context. One COM initialization per thread, reused for the thread's lifetime.
• Bonus: Eliminates duplicated clipboard functions that existed identically in both input_field/mod.rs and text_area/mod.rs.

What changed

File	Change
src/clipboard.rs	New shared module with system_clipboard_set/system_clipboard_get using thread-local caching
src/lib.rs	Register clipboard module (gated on clipboard feature)
src/component/input_field/mod.rs	Replace inline clipboard functions with import from shared module
src/component/text_area/mod.rs	Replace inline clipboard functions with import from shared module
src/component/text_area/update.rs	Update import path

Test plan

• cargo test --all-features --lib passes (4086 tests)
• cargo test --all-features --doc passes (787 tests)
• cargo test --no-default-features --lib passes (793 tests)
• cargo clippy --all-features -- -D warnings clean
• CI passes on Windows (the platform where this manifests)

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 90.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.12%. Comparing base (18a5375) to head (655fa1b).

Files with missing lines	Patch %	Lines
src/clipboard.rs	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #215      +/-   ##
==========================================
+ Coverage   92.08%   92.12%   +0.03%     
==========================================
  Files         145      146       +1     
  Lines       12778    12774       -4     
==========================================
+ Hits        11767    11768       +1     
+ Misses       1011     1006       -5     

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

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.