Show Community Restriction Modal

## Description
Implement a *Community Restriction Modal* on mobile, shown when a user attempts to access content (e.g. a world, scene, or chat) that is gated to members of a specific Decentraland Community. The modal should inform the user of the restriction and offer a path to join the community (if joining is allowed) or clearly explain why access is denied.

This is part of the broader Communities feature effort tracked in #656 (Communities Menu, Discovery, Search, Joining, and Community Text Chat).

## Notes
- No existing reference implementation was found in `unity-explorer` or `godot-explorer` at the time of writing — exact trigger conditions, copy, and design should be confirmed with product/design before implementation.
- Related: #656 (Communities — parent feature, currently "need definition")

## Suggested Scope
- Detect when a user attempts to enter a community-restricted space
- Display modal with restriction messaging
- CTA to join community (if applicable) or dismiss/return

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Requested by Rodri via Slack