Support building grist-core with more than one extension set

Describe the problem to be solved

grist-core currently supports a single set of extensions at build time. grist-desktop adds its own extensions to support Electron, and Full Grist adds a different set. Because the two cannot be combined, features that live in the Full Grist extensions — in particular the built-in MCP server — cannot be included in a Grist Desktop build.

This was confirmed by Paul (Grist CTO) in a recent forum discussion, where he noted there are no plans for a Full Grist edition of Desktop, and that the underlying reason is the lack of support in grist-core for multiple extension sets. "https://community.getgrist.com/t/june-2026-webinar-improved-ai-tooling-with-our-new-mcp-server-and-oauth-connected-apps/14005/4"

A single user working locally on their own laptop, who wants to connect an AI assistant to their Grist documents through the built-in MCP server. Today the only option is to replace Grist Desktop with the full-edition Docker image, which is a heavier setup for someone who chose the desktop app precisely to avoid running a server.

Describe the solution you would like

Allow grist-core to be built with more than one extension set, so that a build could combine the Electron extensions with the Full Grist extensions. This would make a full-edition Grist Desktop build possible, activated with a normal activation key.

This is not a request to make full-edition features free — an activation key would be expected.