feat(platform): add chat export as PDF and Markdown

[larryro]

Summary

• Add export/download button to the chat header (desktop + mobile) next to the Share button
• Export dialog with message selection: all messages checked by default, users can deselect individual messages or toggle select/deselect all
• Two export formats: PDF (via browser print dialog) and Markdown (instant client-side download)
• Internal structural markers ([[CONCLUSION]], [[KEY_POINTS]], [[DETAILS]], etc.) are stripped from exported content
• i18n support for English and German

Closes #1189

Test plan

• Open a chat with messages, click "Export" button in the header
• Verify all messages are selected by default in the dialog
• Deselect some messages, click "Download PDF" — verify browser print dialog opens with only selected messages
• Verify Chinese/CJK text renders correctly in the PDF (uses system fonts)
• Click "Download Markdown" — verify .md file downloads with correct content
• Verify "Select all" / "Deselect all" toggle works
• Verify export button appears on both desktop and mobile layouts
• Verify exported content does not contain [[CONCLUSION]], [[KEY_POINTS]] markers

Summary by CodeRabbit

Release Notes

• New Features
  • Chat export feature now available—download conversations as Markdown or PDF format files
  • Selectively choose which messages to include in your export with checkbox controls
  • Option to select or deselect all messages with a single action
  • Available in English and German language interfaces

[greptile-apps]

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[coderabbitai]

Caution

Review failed

Pull request was closed or merged during review

📝 Walkthrough

Walkthrough

This pull request adds a chat export feature to the platform. A new "Download" button is added to the chat header component that opens an export dialog. The dialog displays selectable messages (user and assistant roles only), allows users to select or deselect individual messages or all at once, and provides two export options: Markdown and PDF formats. Supporting changes include a new Convex query endpoint for fetching thread messages, and localization strings in English and German for all export-related UI labels and messaging.

Estimated code review effort

🎯 4 (Complex) | ⏱️ ~60 minutes

Possibly related PRs

• fix(platform): UI polish and layout fixes #417: Modifies the same chat header component (services/platform/app/features/chat/components/chat-header.tsx) to adjust styling and active-state logic that may interact with the new export button placement.
• feat(platform): add chat message editing and conversation branching #1271: Implements message branching and editing functionality that also relies on fetching thread messages via the Convex backend, and would integrate with the new getThreadMessages query introduced here.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main feature addition: a chat export capability supporting PDF and Markdown formats, which directly matches the primary changes in the changeset.
Linked Issues check	✅ Passed	The PR fully implements the requirement from issue #1189 by providing an export feature for AI-generated chat results in both PDF and Markdown formats, meeting the documented product requirement.
Out of Scope Changes check	✅ Passed	All changes are directly aligned with the export feature scope: UI button and dialog components, backend query for fetching messages, and i18n translations for English and German—no unrelated modifications detected.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/export-chat-pdf-markdown

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