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

backend/internal/server/server.go

@@ -2,6 +2,7 @@ package server
 
 import (
 	"fmt"
+	"io/fs"
 	"net/http"
 	"path"
 	"time"
@@ -9,6 +10,7 @@ import (
 	"github.com/sirupsen/logrus"
 	"reichard.io/aethera/internal/api"
 	"reichard.io/aethera/internal/store"
+	"reichard.io/aethera/web"
 )
 
 func StartServer(settingsStore store.Store, dataDir, listenAddress string, listenPort int) {
@@ -17,12 +19,13 @@ func StartServer(settingsStore store.Store, dataDir, listenAddress string, liste
 	// Create API Instance - use settingsStore as the unified store for both settings and chat
 	logger := logrus.New()
 	api := api.New(settingsStore, dataDir, logger)
-	feFS := http.FileServer(http.Dir("../frontend/public/"))
-	mux.Handle("GET /", feFS)
 
-	// Serve UI Pages
-	pagesFS := http.FileServer(http.Dir("../frontend/public/pages/"))
-	mux.Handle("GET /pages/", http.StripPrefix("/pages/", pagesFS))
+	// Serve embedded static assets
+	staticFS, err := fs.Sub(web.Assets, "static")
+	if err != nil {
+		logrus.Fatal("Failed to create static filesystem: ", err)
+	}
+	mux.Handle("GET /", http.FileServer(http.FS(staticFS)))
 
 	// Serve Generated Data
 	genFS := http.FileServer(http.Dir(path.Join(dataDir, "generated")))