feat(shell): highlight user input with bright blue and separator for better visibility

[liuchong]

Description

Improve visual distinction of user messages in the shell UI by highlighting user input with bright blue color (#007AFF) and adding a full-width separator line below each user input.

Changes

• echo.py:

  • Apply bright blue color (#007AFF) to user echo text for unified visual identity
  • Add a full-width dashed separator line below user input for clear visual separation
  • Update return type from Text to Group to accommodate the new layout

• prompt.py:

  • Add user-input style class with fg:#007AFF to match the terminal icon color

Test Plan

• make check passes (ruff + pyright)
• make test passes (13/13 tests in test_shell_prompt_echo.py)
• Manual test: Verified user input appears in bright blue #007AFF
• Manual test: Verified separator line displays correctly

Checklist

• I have read the CONTRIBUTING document.
• I have tested these changes locally.
• I have run make gen-changelog to update the changelog.

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 4 additional findings.

[liuchong]

@stdrc @RealKai42

[liuchong]

Hi @RealKai42 @sailist @stdrc, could you please take another look at this PR when convenient?

I rebased/rebuilt the branch on top of the current origin/main and removed the previous merge commit. The PR is now a single linear commit:

• ec10f846 feat(shell): highlight user input with separator

Validation run locally:

• uv run pytest tests/ui_and_conv/test_shell_prompt_echo.py tests/ui_and_conv/test_replay.py tests/ui_and_conv/test_shell_run_placeholders.py tests/ui_and_conv/test_visualize_running_prompt.py — 89 passed
• make check-kimi-cli — passed

The remaining diff is limited to the shell user-input echo/style behavior and related tests. Thanks!