feat(build): embed static assets into Go binary

Embed frontend build output directly into Go binary using //go:embed.
This removes runtime dependency on ../frontend/public/ path and
simplifies Docker builds by serving assets from embedded filesystem.

- Add backend/web/embed.go with embed.FS directive
- Update server to serve from embedded static assets
- Update Makefile to copy frontend build to web/static/
- Update Dockerfile for simplified multi-stage build
- Update frontend package.json output paths
- Remove custom 'oc' command from flake.nix dev shell

Makefile

@@ -4,6 +4,9 @@ all: frontend backend
 
 frontend:
 	cd frontend && bun run build
+	mkdir -p backend/web/static
+	cp frontend/public/index.html backend/web/static/ 2>/dev/null || true
+	cp -r frontend/public/pages backend/web/static/ 2>/dev/null || true
 
 backend:
 	cd backend && go build -o ./dist/aethera ./cmd
@@ -11,6 +14,7 @@ backend:
 clean:
 	rm -rf frontend/public/dist
 	rm -rf backend/dist
+	rm -rf backend/web/static
 
 dev:
 	cd backend && go run ./cmd --listen 0.0.0.0 &