Fix: Correctly load template for knowledge card sections

Created by: Edouard-Legoupil

The frontend was failing to display generated sections for existing knowledge cards because it was not loading the correct template file. It was deriving the template name from the link type (e.g., 'donor') instead of using the template_name field stored in the knowledge card's data.

This commit introduces two changes to fix this:

1. Backend: A new API endpoint /api/templates/{template_name} has been added to backend/api/proposals.py to allow fetching a specific template file by its name.

2. Frontend: The KnowledgeCard.jsx component has been updated to:

   • Fetch the template using the template_name from the card data when editing an existing card.
   • Continue to derive the template name from the linkType only when creating a new card.

This ensures that the correct template is always loaded, allowing the generated sections to be displayed correctly.

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