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/app/sitemap.ts

@@ -212,12 +212,6 @@ export default function sitemap(): MetadataRoute.Sitemap {
 			priority: 0.8,
 		},
 		// How-to documentation
-		{
-			url: "https://www.surfsense.com/docs/how-to/electric-sql",
-			lastModified,
-			changeFrequency: "daily",
-			priority: 0.8,
-		},
 		{
 			url: "https://www.surfsense.com/docs/how-to/realtime-collaboration",
 			lastModified,