(SP: 1) [Frontend] Remove Sanity CMS dependencies and delete dead code

[LesiaUKR]

Goal

Eliminate all Sanity dependencies from the codebase. Stop Sanity billing. Clean up dead code.

Scope

Prerequisites: Issues #386, #387, #388 must be merged and verified on production.

Steps:

1. Remove from frontend/package.json:

   • @sanity/client
   • @sanity/image-url
   • groq

2. Delete frontend/client.ts (Sanity client singleton)

3. Remove Sanity env vars from all environments (Vercel, local .env):

   • NEXT_PUBLIC_SANITY_PROJECT_ID (if set)
   • SANITY_DATASET
   • GROQ_API_KEY

4. Keep studio/ directory frozen — do not delete. Sanity free plan keeps data accessible as read-only archive indefinitely. Do not pay for or deploy Sanity Studio.

5. npm run lint — confirm zero import errors

6. npm run build — confirm clean build, no references to removed packages

Verification:

• npm run build completes with no errors
• No occurrences of @sanity/client, sanity, groq in frontend/ (run grep)
• Sanity subscription cancelled/downgraded to free

Expected impact

Sanity billing eliminated. Codebase has zero Sanity dependencies.

Out of scope

• Deleting the Sanity project data (keep as archive)
• Deleting the studio/ directory (keep frozen)