Allow the server root to be configurable

[nvwx]

We have a use case where our server could be accessed via a cloudflare tunnel that directs traffic to the mcp server's port based upon a path. For example: domain.com/webflow connects to localhost:1338

I'd like to request that the root be configurable instead of hard-coded to "/" which causes our current issue of receiving a failure when attempting to connect:

Cannot GET /webflow/callback

Relevant section:

mcp-server/src/modules/designerAppBridge.ts

Line 155 in c0b5084

app.get("/", (_, res) => {