Fix up paths for KV to align with new values

[jasonrclark]

Expected paths for the KV client changed with https://github.com/github/workbench-template/commit/c3b2da9f0e8ad8ca50133cc140729c8e398bcb8c#diff-6a3b01ba97829c9566ef2d8dc466ffcffb4bdac08706d3d6319e42e0aa6890dd but this didn't end up over in spark-template as well.

[Copilot]

Pull Request Overview

This PR updates the KV service endpoints to use the new /_spark prefix for both frontend environment variables and local development proxies.

• Changed BASE_KV_SERVICE_URL from "/kv" to "/_spark/kv"
• Updated dev server proxy paths for KV and LLM to include /_spark prefix

Comments suppressed due to low confidence (2)

vite.config.ts:44

• Consider adding a corresponding BASE_LLM_SERVICE_URL definition for "/_spark/llm" so the LLM service URL is available via environment variables, mirroring the KV change.

BASE_KV_SERVICE_URL: JSON.stringify("/_spark/kv"),

vite.config.ts:74

• Add a rewrite rule to strip the /_spark prefix when proxying to the backend (e.g., rewrite: path => path.replace(/^\/_spark/, '')), unless the backend routes have been updated to include the new prefix.

"/_spark/kv": {