feat(snippet): add editable importance level (1–5)

[derangulacau]

Feature Description

This feature allows users to modify the importance level of a paragraph snippet
through the Modify Data Snippet dialog.

Users can select an importance level (1–5) using radio buttons.
The selected value is saved when the user clicks Done.

Implementation

• Added Angular Material radio buttons (mat-radio-group, mat-radio-button)
  in add-content.component.html
• Introduced importance state handling in add-content.component.ts
• Loaded existing importance when editing a snippet
• Included importance in payload for create and update operations

Result

• Users can edit importance level of snippets
• Importance persists correctly after update
• Improved UX with clear selection interface