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

frontend/package.json

@@ -2,8 +2,8 @@
   "name": "aethera",
   "private": true,
   "scripts": {
-    "dev": "bun build src/main.ts --outdir public/dist --target browser --watch & bunx @tailwindcss/cli -i styles.css -o public/dist/styles.css --watch",
-    "build": "bun build src/main.ts --outdir public/dist --target browser && bunx @tailwindcss/cli -i styles.css -o public/dist/styles.css --minify",
+    "dev": "bun build src/main.ts --outdir public/dist --target browser --watch & bunx tailwindcss -i styles.css -o public/dist/styles.css --watch",
+    "build": "bun build src/main.ts --outdir ../backend/web/static/dist --target browser && bunx tailwindcss -i styles.css -o ../backend/web/static/dist/styles.css --minify",
     "lint": "eslint ./src/**"
   },
   "devDependencies": {