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/composables/mapConfig.js

@@ -5,6 +5,7 @@
 // install still renders; each site uploads its own blueprint in Settings.
 import { reactive } from 'vue'
 import { settingsApi } from '../api'
+import { withBase } from '../utils/basePath'
 
 // Fallback defaults - match the seeded map_blueprint_* setting defaults.
 const DEFAULTS = {
@@ -59,7 +60,7 @@ export function reloadMapConfig() {
 
 // Blueprint image URL for the given theme ('light' | 'dark').
 export function blueprintUrlFor(theme) {
-  return theme === 'light' ? state.blueprintLight : state.blueprintDark
+  return withBase(theme === 'light' ? state.blueprintLight : state.blueprintDark)
 }
 
 export function useMapConfig() {