Upload floor-map blueprints (light/dark) instead of typing a path

The Floor Map settings only had text fields for the blueprint image path with
no way to upload one. Add a real upload.

- POST /settings/map-blueprint (multipart: file + theme) saves the image to the
  instance maps dir and points map_blueprint_<theme> at the served URL;
  GET /settings/map-blueprint/<file> serves it (public - kiosks read the map).
- Settings > Floor Map: a file picker + thumbnail next to each theme's field
  (still accepts a manual path/URL too).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/api/index.js

@@ -793,6 +793,12 @@ export const settingsApi = {
   },
   create(data) {
     return api.post('/settings', data)
+  },
+  uploadMapBlueprint(theme, file) {
+    const form = new FormData()
+    form.append('theme', theme)
+    form.append('file', file)
+    return api.post('/settings/map-blueprint', form, { headers: { 'Content-Type': 'multipart/form-data' } })
   }
 }