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/src/utils/basePath.js

@@ -0,0 +1,13 @@
+// Single source for the app's mount path. Vite injects import.meta.env.BASE_URL
+// from the build-time `base` (default '/', or e.g. '/ops/' for a subpath IIS
+// mount). Every root-absolute URL to a Flask-served asset or route must go
+// through withBase() so it resolves under the mount instead of the server root.
+export const BASE_URL = import.meta.env.BASE_URL
+
+// Prefix a root-absolute path (e.g. '/ge-aerospace-logo.svg', '/api', '/tv')
+// with the mount base. Leaves full URLs (http, data:) untouched.
+export function withBase(path) {
+  if (!path) return path
+  if (/^([a-z]+:)?\/\//i.test(path) || path.startsWith('data:')) return path
+  return BASE_URL + String(path).replace(/^\//, '')
+}