refactor: remove PostCategory

[matiasleyba]

Description

As part of #1348, categories will be transitioned from being an enum to an object class. This change allows categories to be dynamically defined by the backend.
As a result, the PostCategory enum has been removed since its values will no longer align with dynamically defined categories.

Key Changes

• API
  The PostCategory enum is removed and replaced with the category id to align with the dynamic structure.

• App
  Any UI Block that previously relied on PostCategory now includes a categoryId to support dynamic categories.
  The name of the category is resolved using the CategoriesBloc state, which stores all fetched categories.

• Bloc Placement:
  The CategoriesBloc has been moved higher in the widget tree to make its data accessible across different screens, such as the ArticleDetailsPage.

• Hydrated Bloc Update:
  The CategoriesBloc is no longer a hydrated bloc because categories are dynamic and storing them locally is unnecessary. The backend might change or update them at any time.

📝 Note
The refactor of Category into an object class will be handled in a subsequent PR. This PR focuses first on refactoring PostCategory, with the Category refactor to follow later, ensuring the changes remain manageable and the PR size is kept reasonable.

Type of Change

• ✨ New feature (non-breaking change which adds functionality)
• 🛠️ Bug fix (non-breaking change which fixes an issue)
• ❌ Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 Code refactor
• ✅ Build configuration change
• 📝 Documentation
• 🗑️ Chore

[marwfair]

LGTM!