Create apiBase module and wire client fetch to /api/*

[kmcginnes]

Parent PRD

• Parent Simplify client-server relationship by removing proxy URL from connection config #1618

What to build

Create a new module exporting an apiBase constant computed from import.meta.env.BASE_URL. Update all explorer fetch functions (_gremlinFetch, _sparqlFetch, _openCypherFetch) and summary endpoints to use apiBase instead of connection.url. Update the Vite dev server proxy config to a single /api rule.

This is the core client-side change that decouples outbound requests from the connection URL.

Type: HITL — requires review before completion.

Acceptance criteria

• New apiBase module exports a constant derived from BASE_URL
• apiBase resolves to http://localhost/api/ for default BASE_URL of /
• apiBase resolves correctly for prefixed BASE_URL (e.g., /proxy/9250/)
• All explorer fetch functions use apiBase for request URLs
• Summary endpoints use apiBase for request URLs
• Vite dev proxy config simplified to "/api": { target: expressServerUrl }
• Tests verify apiBase computation for default and prefixed BASE_URL
• Tests verify explorer fetch functions send requests to correct apiBase-derived URLs

Blocked by

• Blocked by Move Express API routes under /api/ prefix #1619

User stories addressed

• User story 8
• User story 12