✨(frontend) add markdown download option

[gokul-debugger]

Purpose

Add Markdown to the document download menu so users can export document
content directly as a .md file.

Closes #2605.

Proposal

• Reuse the existing Markdown conversion logic
• Download the file using the document title
• Add Markdown to the download dropdown
• Update the supported export formats message
• Add focused coverage for Markdown downloads

External contributions

Thank you for your contribution! 🎉

Please ensure the following items are checked before submitting your pull request:

General requirements

• I have read and followed the contributing guidelines
• I have read and agreed to the Code of Conduct
• I have added corresponding tests for new features or bug fixes (if applicable)

Skip the checkbox below 👇 if you're fixing an issue or adding documentation

• Before submitting a PR for a new feature I made sure to contact the product manager

CI requirements

• I made sure that all existing tests are passing
• I have signed off my commits with git commit --signoff (DCO compliance)
• I have signed my commits with my SSH or GPG key (git commit -S)
• My commit messages follow the required format: <gitmoji>(type) title description
• I have added a changelog entry under ## [Unreleased] section (if noticeable change)

AI requirements

• I used AI assistance to produce part or all of this contribution
• I have read, reviewed, understood and can explain the code I am submitting
• I can jump in a call or a chat to explain my work to a maintainer

Tool used: ChatGPT, for codebase understanding and implementation review.

Testing

• Frontend lint suite passed
• App unit suite passed: 300 tests
• Production frontend build passed
• Focused Chromium Playwright E2E passed: Markdown export, filename, and downloaded content verified

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 021bf2e0-38e9-448e-b3a0-cf401c449c6b

📥 Commits

Reviewing files that changed from the base of the PR and between 944ea13 and bcbe755.

📒 Files selected for processing (3)

• CHANGELOG.md
• src/frontend/apps/e2e/__tests__/app-impress/doc-export.spec.ts
• src/frontend/apps/impress/src/features/docs/doc-export/components/ModalExport.tsx

Included review availability: Your plan provides up to 10 included reviews per hour; 9 remain after this review.

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Walkthrough

The document export modal now includes Markdown as an available format. It generates a UTF-8 Markdown Blob when the configured exporter returns no Blob and uses the .md extension for downloads. The end-to-end coverage selects Markdown, checks the filename, and verifies the downloaded content. The changelog records the new frontend download option.

Estimated code review effort: 2 (Simple) | ~10 minutes

Merge Risk: ⚪ Minimal · up to bcbe7

This PR adds a browser-local Markdown download option using the existing export flow and does not broaden document access or introduce service changes. No actionable merge-blocking risk remains beyond normal checks and review.

Suggested reviewers: antolc, ovgodd

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 2 files. (1 skipped: 1 …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The changes satisfy issue #2605 by adding a Markdown download option, reusing document Markdown conversion, producing UTF-8 .md files, and adding focused coverage.
Out of Scope Changes check	✅ Passed	The changelog update, export modal change, and end-to-end test are directly related to the Markdown download objective. No unrelated changes are identified.
Title check	✅ Passed	The title clearly and concisely describes the main change: adding a Markdown download option to the frontend.
Description check	✅ Passed	The description directly explains the Markdown export feature, implementation proposal, testing, and related issue.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 1 functions across 2 files. (1 skipped: 1 unsupported.)

• Fix all pre-merge checks with AI

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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