TUI: add reader-local copy and export actions

[zhnt]

Summary\n\nTrack a deferred TUI follow-up for transcript reader-local copy/export actions.\n\n## Scope\n\n- Add reader-local copy/export semantics for the currently rendered transcript reader content.\n- Preserve raw/detail mode semantics when copying or exporting from the reader.\n- Consider search-aware copy behavior, such as copying the full reader body while search highlighting remains visual-only.\n- Add focused unit and playback coverage for reader copy/export behavior.\n\n## Non-goals\n\n- Do not introduce mouse or screen-buffer selection in this issue.\n- Do not change code/method lane behavior.\n- Do not require exact compatibility with pi.tui visuals; judge semantic completeness.\n\n## Notes for agents and reviewers\n\nThis should build on the existing transcript reader source/copy boundary work. Keep the implementation scoped to TUI/product adapter/playback/transcript reader files. Validate that copied/exported content is plain text without ANSI search-highlight sequences unless an explicit styled export mode is introduced.\n\nRelated design doc: docs/internals/architecture/tui/native-terminal-core/key-designs/KD-018-transcript-reader-and-copy-semantics.md