feat: floor plan viewport rendering, per-canvas config, server media upload

- Render floor plan inside React Flow ViewportPortal so it pans/zooms with
  nodes (was screen-fixed, desynced on pan/zoom); zoom-stable resize handles.
- Move floor plan config from the left panel into the canvas (design) edit
  modal; attach per-design and fix cross-design bleed on load.
- Store images via a new generic backend media endpoint (POST/GET/DELETE
  /api/v1/media) on disk under <data_dir>/uploads, not base64 in the canvas.
- Disable floor plans in standalone mode (no backend to upload/serve); drop
  base64 localStorage persistence. See ADR-001 in CLAUDE.md.
- Tests: backend media route, DesignModal floor plan + upload, store floorMap.

ha-relevant: maybe

frontend/src/api/client.ts

@@ -41,6 +41,17 @@ export const canvasApi = {
   }) => api.post('/canvas/save', payload),
 }
 
+export const mediaApi = {
+  /** Upload an image, returns its server URL (e.g. /api/v1/media/<uuid>.png). */
+  upload: async (file: File): Promise<{ url: string; filename: string }> => {
+    const form = new FormData()
+    form.append('file', file)
+    const res = await api.post<{ url: string; filename: string }>('/media/upload', form)
+    return res.data
+  },
+  delete: (filename: string) => api.delete(`/media/${filename}`),
+}
+
 export const nodesApi = {
   create: (data: object) => api.post('/nodes', data),
   update: (id: string, data: object) => api.patch(`/nodes/${id}`, data),