Make the image build from a clean clone and actually serve the interface

The Dockerfile copied PRD.md, which is not in the repository, so a clean
clone could not build at all. It also installed node and npm from Ubuntu 24
(node 18) to build a Vite 8 front end that needs node 20.19 or newer, and it
built the interface into a directory nothing served.

The build is now two stages: node:22 builds the interface, the CUDA runtime
image installs the package and carries the built files. The API serves them
when ELY_EYE_UI_DIR points at a directory that exists, and refuses to start
when it is set but empty, naming the build command. The front end already
calls /api/... on its own origin, so same-origin serving needs no client
change.

backend/src/ely_eye/api.py

@@ -7,6 +7,7 @@ from typing import Annotated
 
 from fastapi import Depends, FastAPI, File, HTTPException, UploadFile
 from fastapi.middleware.cors import CORSMiddleware
+from fastapi.staticfiles import StaticFiles
 from pydantic import BaseModel
 
 from . import __version__
@@ -339,6 +340,15 @@ def create_app() -> FastAPI:
             "cartridges": CartridgeService(settings, db).list_manifests(),
         }
 
+    mounted_ui = get_settings().ui_dir
+    if mounted_ui is not None:
+        if not mounted_ui.is_dir():
+            raise SystemExit(
+                f"ui_dir {mounted_ui} is set but holds no built interface; "
+                "run `npm run build` in frontend/ or unset ELY_EYE_UI_DIR"
+            )
+        app.mount("/", StaticFiles(directory=mounted_ui, html=True), name="ui")
+
     return app