feat [SCRUM-28]: implement PDF export with node selection and AI summary support

[Heshan1478]

Summary

This PR implements the "Export as PDF" feature for the frontend. Users can now select specific chat nodes and export them as a structured PDF document, with optional inclusion of AI-generated summaries.

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What Changed

PDF Export Implementation

• Integrated PDF generation using html2canvas and jsPDF
• Replaced previous html2pdf.js approach with a more flexible rendering method
• Generates downloadable PDF file (collee-export-YYYY-MM-DD.pdf)

Node Selection Workflow

• Introduced selection mode for exporting nodes
• Users can:
  • Select multiple nodes
  • Include/exclude AI summaries per node
• Automatically preselects the clicked node when export is initiated

UI Enhancements

• Added node-level ⋯ menu in IdeaNode
  • Export as PDF (functional)
  • Export as Text (placeholder)
• Added checkboxes for:
  • Node selection
  • AI summary inclusion (only when available)
• Implemented click-outside behavior for menu

Sticky Action Bar

• Displays during export selection mode
• Includes:
  • "Select nodes to export" label
  • Cancel button
  • Export PDF button

Data Handling

• Iterates selected nodes in canvas order
• Includes:
  • Node title
  • Chat messages
  • AI summary section (if selected)

State Management

• Centralized selection state in CanvasWorkspace
• Clean exit handling:
  • Reset selection on cancel/export
  • Exit selection mode after export

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🔗 SCRUM Ticket

https://forkly.atlassian.net/browse/SCRUM-28?atlOrigin=eyJpIjoiNjYwZjY0NDU4OGY3NGQzMjhiYWM4MjRkOTAyODA1MDgiLCJwIjoiaiJ9

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Testing Steps

1. Open the chat canvas
2. Click ⋯ on any node
3. Select "Export as PDF"
4. Verify:
   • Selection mode is activated
   • Node is preselected
   • Summary checkbox appears if available
5. Select multiple nodes
6. Click "Export PDF"
7. Confirm:
   • PDF downloads successfully
   • Contains correct nodes and content
8. Click "Cancel" and ensure state resets correctly

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Checklist

• Code follows the project's style guidelines
• Self-reviewed my own code
• Added/updated comments where necessary
• No new warnings or errors introduced
• Tested locally with docker compose up --build
• All CI checks pass

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