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).

deploy/windows/web.config

@@ -38,6 +38,13 @@
              config (SQL echo, debug, wrong DB URL). Real secrets go in .env. -->
         <environmentVariable name="FLASK_ENV" value="production" />
         <environmentVariable name="PYTHONPATH" value="C:\shopdb-flask" />
+        <!-- Subpath method only: when this web.config sits in an IIS
+             Application (e.g. /ops) under an existing site instead of its own
+             site, tell the app its mount path. Must match the alias the
+             Application was created with AND the VITE_BASE_PATH the frontend
+             was built with ('/ops/'). Omit for the own-site method.
+        <environmentVariable name="MOUNT_PATH" value="/ops" />
+        -->
       </environmentVariables>
     </httpPlatform>