fix(tui): use full-height prompt marker

[svadrutk]

Summary

• replace the short Unicode › composer marker with a full-height ASCII >
• keep the existing one-cell gutter padding beside the marker
• assert the marker in both the initial welcome composer and the compact post-message composer

The larger first-message card does not need a stretched or repeated marker: its extra rows are reserved for the provider/model metadata and spacing. The marker is rendered only in the editable row, so it stays aligned with the input text in both layouts.

Automated tests

• cargo fmt --check
• cargo test -q tui::ui::tests

Manual test

1. Run cargo run -- tui --root /path/to/a/test/project from this branch.
2. On the welcome screen, confirm the > matches the input text's line height, has a space before message kit…, and does not extend into the metadata row.
3. Send a message, then confirm the compact composer uses the same full-height > with the rounded border providing vertical padding.
4. Enter a long or multiline prompt and confirm wrapping, cursor movement, and the marker's alignment with the first visible input row still work.

[kit-code-agent]

No issues found. The changes look good to merge.