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/content/docs/docker-installation/dev-compose.mdx

@@ -25,6 +25,5 @@ The following `.env` variables are **only used by the dev compose file** (they h
 | `NEXT_PUBLIC_FASTAPI_BACKEND_AUTH_TYPE` | Frontend build arg for auth type | `LOCAL` |
 | `NEXT_PUBLIC_ETL_SERVICE` | Frontend build arg for ETL service | `DOCLING` |
 | `NEXT_PUBLIC_DEPLOYMENT_MODE` | Frontend build arg for deployment mode | `self-hosted` |
-| `NEXT_PUBLIC_ELECTRIC_AUTH_MODE` | Frontend build arg for Electric auth | `insecure` |
 
 In the production compose file, the `NEXT_PUBLIC_*` frontend variables are automatically derived from `AUTH_TYPE`, `ETL_SERVICE`, and the port settings. In the dev compose file, they are passed as build args since the frontend is built from source.