fix(help): handle Ctrl+H as delete on some systems (#958)

[JiarenWang]

Summary

Fixes #958.

• treat Ctrl+H the same as Backspace in the help overlay filter input
• document the terminal compatibility behavior inline in the help view implementation
• add a regression test covering Ctrl+H widening the filtered result set

Testing

• cargo test -p deepseek-tui ctrl_h_widens_match_set
• cargo test --all-features
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features

Checklist

• Updated docs or comments as needed
• Added or updated tests where relevant
• Verified TUI behavior manually if UI changes

[gemini-code-assist]

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