chore: remove Electric SQL plumbing and infrastructure

Remove all Electric SQL client code, Docker service, env vars, CI build
args, install scripts, and documentation. Feature hooks that depend on
Electric are intentionally left in place to be rewritten with Rocicorp
Zero in subsequent commits.

Deleted:
- lib/electric/ (client.ts, context.ts, auth.ts, baseline.ts)
- ElectricProvider.tsx
- docker/scripts/init-electric-user.sh
- content/docs/how-to/electric-sql.mdx

Cleaned:
- package.json (4 @electric-sql/* deps)
- app/layout.tsx, UserDropdown.tsx, LayoutDataProvider.tsx
- docker-compose.yml, docker-compose.dev.yml
- Dockerfile, docker-entrypoint.js
- .env.example (frontend, docker, backend)
- CI workflows, install scripts, docs

surfsense_web/.env.example

@@ -2,10 +2,6 @@ NEXT_PUBLIC_FASTAPI_BACKEND_URL=http://localhost:8000
 NEXT_PUBLIC_FASTAPI_BACKEND_AUTH_TYPE=LOCAL or GOOGLE
 NEXT_PUBLIC_ETL_SERVICE=UNSTRUCTURED or LLAMACLOUD or DOCLING
 
-# Electric SQL
-NEXT_PUBLIC_ELECTRIC_URL=http://localhost:5133
-NEXT_PUBLIC_ELECTRIC_AUTH_MODE=insecure
-
 # Contact Form Vars - OPTIONAL
 DATABASE_URL=postgresql://postgres:[YOUR-PASSWORD]@db.sdsf.supabase.co:5432/postgres