Support subpath IIS deployment as a second install method

The app can run as an IIS Application under an existing site
(e.g. https://host/ops/) instead of its own site + port:

- frontend: vite base via VITE_BASE_PATH; router history, axios
  baseURL, and root-absolute asset/route paths resolve through
  utils/basePath.js withBase()
- backend: MOUNT_PATH (env or .env) wraps the app in a WSGI
  middleware that shifts the prefix into SCRIPT_NAME, so one knob
  serves API + SPA under the mount
- docs: INSTALL-WINDOWS-IIS.md section 7b runbook + troubleshooting
  rows; DEPLOY-WINDOWS-IIS.md pointer; commented examples in
  deploy/windows/web.config and .env.example

Root deployment unchanged (MOUNT_PATH unset, base '/'). Also folds
two stray root-absolute callers into the shared plumbing
(MachineForm relationship-types fetch, reports CSV window.open).

docs/DEPLOY-WINDOWS-IIS.md

@@ -128,6 +128,13 @@ to reproduce the exact enabled set, then just run `flask plugin upgrade-all`.)
 
 ## 6. Create the IIS site + web.config
 
+This describes the own-site method (the app gets its own IIS site + port). To
+mount the app at a subpath under an existing site instead (e.g.
+`https://<host>/ops/` sharing the classic site's binding and cert), see
+**docs/INSTALL-WINDOWS-IIS.md section 7b**: same web.config, but the site is a
+`New-WebApplication` under the parent, `MOUNT_PATH=/ops` is set (web.config or
+`.env`), and the frontend is built with `VITE_BASE_PATH=/ops/`.
+
 1. In IIS Manager, add a new **Site** (separate from the classic ASP site):
    - Physical path: `APP_ROOT`
    - Binding: a free port or a dedicated hostname (e.g. `https` 443 with the