Improve Category Management and Support Multiple Categories for Items

Is your feature request related to a problem? Please describe.
Currently, administrators must manually remember category assignments or search through the frontend to understand which categories are configured for a given component.
This is inefficient and error-prone.

Furthermore, proposals or participatory items often relate to multiple topics—for example, SDGs-related proposals may apply to several goals—but Decidim only allows selecting one category. This limits expressiveness and makes classification difficult.

Describe the solution you'd like
I would like:

1. An admin-side overview page listing all categories defined for a component or process, so administrators can quickly see:

   • category names

   • hierarchy

   • usage status

   • related components

2. The ability to assign multiple categories to a proposal or other participatory content types.
   Example: An SDGs-related proposal may belong to Goal 11 and Goal 13, but the current one-category limitation prevents accurate classification.

Describe alternatives you've considered

• Using tags instead of categories, but tags do not support hierarchical organization.

• Creating overly granular categories to simulate "multi-category" selection, which becomes confusing for participants.

• Adding custom fields, but this is not aligned with the existing category model.

Additional context
Multi-category support would benefit any process dealing with cross-cutting policy areas such as:

• climate action

• gender equality

• public health

• strategic planning (e.g., SDGs)

Many public-sector users need multi-dimensional categorization to accurately reflect real-world issues.

Does this issue could impact on users private data?
No.
The request concerns metadata for categorization and admin usability. No private user data is involved.