Feat: add live subject preview for interactive commit (--preview)

[ttw225]

#1901
The implementation is based on Questionary's bottom_toolbar

Description

• Add --preview flag (and config preview) to show a live first-line (subject) preview while typing during cz commit.
• Display a bottom toolbar with subject preview + character counter; wrap by terminal display width (handles CJK width).
• Add a subject length validator that respects message_length_limit (0 disables limit).

Checklist

• I have read the contributing guidelines

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: [Cursor] following the guidelines

Code Changes

• Add test cases to all the changes you introduce
• Run uv run poe all locally to ensure this change passes linter check and tests
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
• Update the documentation for the changes

Documentation Changes

• Run uv run poe doc locally to ensure the documentation pages renders correctly
• Check and fix any broken links (internal or external)

Expected Behavior

• Running cz commit --preview shows a live subject preview and a character counter while answering interactive questions.
• Existing behavior remains unchanged when preview is disabled (default).

Steps to Test This Pull Request

Manual:

• cz commit --preview (observe live subject preview + counter)
• cz commit --preview -l 72 (confirm counter shows current/72 and validation triggers when exceeded)

Additional Context

• New modules: commitizen/interactive_preview.py, commitizen/preview_questions.py
• CLI/config additions: commitizen/cli.py adds --preview; commitizen/defaults.py adds preview setting (default False)
• Docs demos: docs/images/cli_interactive/commit_preview.gif, docs/images/cli_interactive/commit_preview_length_limit.gif (+ .tape)

[codecov]

Codecov Report

❌ Patch coverage is 93.07692% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.79%. Comparing base (33223b4) to head (f18c7ae).
⚠️ Report is 117 commits behind head on master.

Files with missing lines	Patch %	Lines
commitizen/question.py	0.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1902      +/-   ##
==========================================
- Coverage   97.99%   97.79%   -0.20%     
==========================================
  Files          60       62       +2     
  Lines        2689     2811     +122     
==========================================
+ Hits         2635     2749     +114     
- Misses         54       62       +8     

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