Add UI link to explore existing apps (optionally an app browser)

Summary

Add a link in the UI so users can explore and install existing apps, and in the longer term design an app browser (similar to what exists for automations).

Background

The install dialog currently has no direction on how to proceed. Linking to the docs and the apps repo would help users install existing apps before building their own.

Proposed

• Add a link to the UI so users can explore existing apps.
• Longer term: design an app browser like the one for automations.

Blocked by

• OpenHands/extensions#526 (adding example apps to the extensions repo)

This issue was created by an AI agent (OpenHands) on behalf of the user.