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).

frontend/vite.config.js

@@ -3,6 +3,10 @@ import vue from '@vitejs/plugin-vue'
 import path from 'path'
 
 export default defineConfig({
+  // Mount path of the built app. Default '/' (own IIS site / dev). Set
+  // VITE_BASE_PATH='/ops/' (trailing slash) to build for a subpath mount under
+  // Default Web Site, e.g. `VITE_BASE_PATH=/ops/ npm run build`.
+  base: process.env.VITE_BASE_PATH || '/',
   plugins: [vue()],
   resolve: {
     alias: {