feat: configurable UI base path via BASE_PATH build arg (#58)

Serves the UI under /ui instead of /. This pairs with the argocd route simplification (argocd-apps#201) where /ui → UI service and everything else → API.

- Vite: `base` set from `BASE_PATH` env var at build time
- React Router: `basename` set from injected `__BASE_PATH__`
- Nginx: location block uses `${BASE_PATH}`, substituted by sed at build
- Dockerfile: `ARG BASE_PATH=/` (default preserves existing behavior)
- Woodpecker: passes `BASE_PATH=/ui` to docker-web build

Tested: assets serve at `/ui/assets/...`, SPA routing works at `/ui/remotes`, etc.
Reviewed-on: #58
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

.woodpecker/docker.yaml

@@ -22,6 +22,8 @@ steps:
       repo: git.unkin.net/unkin/artifactapi-ui
       dockerfile: ui/Dockerfile.ui
       context: ui
+      build_args:
+        - BASE_PATH=/ui
       username: droneci
       password:
         from_secret: DRONECI_PASSWORD