fix: use gateway-supplied MCP endpoint URL instead of window.location.origin

[marekdano]

Fixes IBM/mcp-context-forge#6632

Depends on IBM/mcp-context-forge#6656 — please don't merge this before that backend PR lands. VirtualServer.url only exists in gateway responses once #6656 ships; until then this PR is a no-op and silently keeps today's window.location.origin fallback behavior.

Summary

Flagged in review on #101:

getVirtualServerEndpoint (src/components/gateways/utils.ts:21-27) still builds the virtual server's MCP endpoint as ${window.location.origin}/servers/{id}/mcp, the same assumption about who owns the origin that this PR removes from redirect_uri. The BFF proxies only /api/*, so that URL hits the SPA fallback and returns index.html with a 200.

Same class of bug as the OAuth redirect_uri fix in #101: the web UI's own origin isn't necessarily where the gateway serves its endpoints in a split deployment. IBM/mcp-context-forge#6656 adds a url field to ServerRead, computed server-side from the gateway's own APP_DOMAIN (the same base URL OAuth's redirect_uri default already uses). This PR switches the frontend to use it.

Changes

• src/types/server.ts: add url?: string to VirtualServer.
• src/components/gateways/utils.ts: getVirtualServerEndpoint now takes the server object and prefers server.url over constructing from window.location.origin, which remains only as a fallback for gateways older than the url field.
• src/components/gateways/VirtualServerDetailsPanel.tsx: updated the one call site.
• Tests: utils.test.ts covers the url-preferred path and both fallback cases.