fix(mcp): honor public OAuth origin

.env.example

@@ -63,6 +63,8 @@ UNSAFE_ENABLE_MOCK_INTEGRATION=true
 
 # Public URL contract. Values must be absolute HTTP(S) URLs without credentials,
 # query strings, or fragments. Trailing slashes are normalized at startup.
+# Public Homarr origin used by MCP OAuth metadata and redirects behind proxies.
+# BASE_URL=https://homarr.example.com
 HOMARR_WEBSITE_URL=https://homarr.dev
 # PocketBase API used by Homarr and by the standalone documentation site.
 # Defaults to HOMARR_WEBSITE_URL.