feat(mcp-servers): resolve store icons from icon_domain via the logo.dev proxy

The MCP store backend is replacing bundled icon assets with an icon_domain
field per template, resolved at render time through the authenticated
mcp_servers/icon/ proxy (PostHog/posthog#70159). Stop reading icon_key
everywhere so the backend can drop the field:

- api-client: add icon_domain to the generated MCP types and a
  getMcpServerIconUrl method (authenticated fetch -> object URL; the
  endpoint's 404 means "no icon", surfaced as null).
- core: iconDomainFromServerUrl derives a best-effort brand domain from a
  server URL (strips mcp./api./www.) so custom installs get icons too;
  resolveServerDetails now resolves iconDomain + serverUrl.
- ui: ServerIcon fetches through the proxy with the active theme, latching
  failures per (domain, theme); bundled assets and BRAND_ICONS removed
  (claude.svg stays, used by ContinueCliSessions).
- mobile: same migration on the hand-mirrored types and ServerIcon (RN Image
  with Authorization header); bundled assets and SERVER_LOGOS removed.

Generated-By: PostHog Code
Task-Id: 2f81c64d-a1c4-483e-affd-a9b355756737

Author: cvolzer3