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/Dockerfile

@@ -35,15 +35,11 @@ RUN corepack enable pnpm
 ARG NEXT_PUBLIC_FASTAPI_BACKEND_URL=__NEXT_PUBLIC_FASTAPI_BACKEND_URL__
 ARG NEXT_PUBLIC_FASTAPI_BACKEND_AUTH_TYPE=__NEXT_PUBLIC_FASTAPI_BACKEND_AUTH_TYPE__
 ARG NEXT_PUBLIC_ETL_SERVICE=__NEXT_PUBLIC_ETL_SERVICE__
-ARG NEXT_PUBLIC_ELECTRIC_URL=__NEXT_PUBLIC_ELECTRIC_URL__
-ARG NEXT_PUBLIC_ELECTRIC_AUTH_MODE=__NEXT_PUBLIC_ELECTRIC_AUTH_MODE__
 ARG NEXT_PUBLIC_DEPLOYMENT_MODE=__NEXT_PUBLIC_DEPLOYMENT_MODE__
 
 ENV NEXT_PUBLIC_FASTAPI_BACKEND_URL=$NEXT_PUBLIC_FASTAPI_BACKEND_URL
 ENV NEXT_PUBLIC_FASTAPI_BACKEND_AUTH_TYPE=$NEXT_PUBLIC_FASTAPI_BACKEND_AUTH_TYPE
 ENV NEXT_PUBLIC_ETL_SERVICE=$NEXT_PUBLIC_ETL_SERVICE
-ENV NEXT_PUBLIC_ELECTRIC_URL=$NEXT_PUBLIC_ELECTRIC_URL
-ENV NEXT_PUBLIC_ELECTRIC_AUTH_MODE=$NEXT_PUBLIC_ELECTRIC_AUTH_MODE
 ENV NEXT_PUBLIC_DEPLOYMENT_MODE=$NEXT_PUBLIC_DEPLOYMENT_MODE
 
 COPY --from=deps /app/node_modules ./node_modules