Admin hub page should hide tiles for modules that aren't registered

[antosubash]

The /admin landing page (Hub) shows tile links for Tenants (/tenants/manage), Pages (/pages/manage), Email Templates (/email/templates), Email History (/email/history), Menus (/settings/menus), Feature Flags (/feature-flags/manage), Rate Limiting (/rate-limiting/manage), Background Jobs (/admin/jobs), Audit Logs (/audit-logs/browse).

In a minimal install (Admin + Users + Permissions + Settings + OpenIddict + Localization), only Users / Roles / OAuth Clients / App Settings actually resolve. The other 5+ tiles 404. The hub UI presents itself as if all peer modules are installed.

Fix options

1. Conditional rendering: inject IServiceProvider (or peer-module marker services) into the Hub view's payload and emit only tiles whose target service exists.
2. Module contributes its own tile: each module that ships an admin page contributes its own tile via a discoverable convention (similar to IModuleMenu → MenuSection.AdminHub), and Admin's hub renders the union.

Option 2 is the cleaner long-term shape — keeps Admin from knowing about every peer module by name, and lets downstream apps add tiles for their own admin pages.

[antosubash]

Resolved by #139.

HubEndpoint.cs injects EndpointDataSource and filters its CandidateUrls against the routes actually registered in the running application. The resulting availableUrls array is passed to the React page, and Hub.tsx filters the groups array so any tile whose URL has no matching endpoint is hidden — and any group that ends up empty is dropped. A minimal install no longer surfaces 404 tiles.

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