fix(tui): copy transcript selections without visual wraps

[knqiufan]

Summary

• Fixes TUI: terminal-native copy includes visual line breaks from wrapped output #1853 by carrying copy-specific wrap metadata through markdown rendering and the transcript cache.
• TUI-owned mouse selection now removes visual wrap breaks while preserving logical newlines, long words, code lines, and list continuation text.
• Documents that raw terminal selection can still include visual wraps when mouse capture is bypassed.

Test plan

• cargo test -p deepseek-tui --bin deepseek-tui selection_to_text
• cargo test -p deepseek-tui --bin deepseek-tui mouse_selection_autocopies_on_release_without_ctrl_c
• cargo clippy -p deepseek-tui --all-targets --all-features
• Manual TUI check: with --mouse-capture, drag-copying a long assistant paragraph pastes as one logical paragraph.

Notes

• cargo test -p deepseek-tui --bin deepseek-tui currently fails on unrelated config/skills tests in this local environment: change_in_non_english_without_api_key_uses_explicit_fallback, test_list_skills_empty_directory, test_list_skills_filters_by_name_prefix, and app_new_detects_missing_api_key_with_default_config.

[gemini-code-assist]

Code Review

This pull request enhances the TUI's copy-paste behavior by introducing metadata to distinguish between visual and logical line breaks. By tracking CopyLineSeparator and copy_prefix_width throughout the rendering pipeline, the selection logic can now remove soft-wrap newlines and strip visual prefixes from code blocks and lists, ensuring the copied text matches the source content. The changes include updates to history rendering, markdown processing, and the selection-to-text logic, along with new unit tests and documentation updates. I have no feedback to provide.