MCP Servers list: type badges, URL display, sorting, and custom filter

- Motivation:
Improve clarity and discoverability in the admin MCP Servers list: visually distinguish proxied remote vs virtual, surface server URL for proxied servers, add sorting beyond creation date, and filter to "custom" servers.

- Proposed solution:
Implement in `frontend/src/app/(dashboard)/mcp-servers/page.tsx`:
- Show server type badges (e.g., Proxied, Virtual) using backend `MCPServerTransportType`.
- Display MCP URL on cards/details for proxied servers.
- Add sorting controls (name, last synced, type).
- Add a "Custom only" filter.

- Additional context:
- Hooks/API: `frontend/src/features/mcp/hooks/use-mcp-servers.ts`, `frontend/src/features/mcp/api/mcp.service.ts`.
- Detail page can also show URL and type badge.