Embed frontend dist into Go binary; remove frontend path from health

- New backend/embed package embeds frontend dist/ via //go:embed
- Go binary serves frontend from embedded FS (no external files needed)
- Dev mode uses placeholder dir (dev.txt) so Go compiles; Vite handles
  frontend in dev mode
- Makefile copies frontend/dist/ to backend/embed/dist/ during build
- Removed FRONTEND_DIR env var and filesystem serving fallback
- Removed frontend path from Health API response and Status page frontend
  section (only Served boolean remains)
- Removed frontendDir from Handler struct/constructor (unused)

.gitignore

@@ -1,6 +1,7 @@
 node_modules/
 dist/
 frontend/dist/
+backend/embed/dist/
 
 *.exe
 *.test