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/types/index.ts

@@ -254,6 +254,11 @@ export interface CustomStyleDef {
 }
 
 export interface FloorMapConfig {
+  /**
+   * Server URL of the uploaded image (e.g. /api/v1/media/<uuid>.png).
+   * Legacy canvases may still hold a base64 `data:` URL — both render in <img>.
+   * Floor plans require a backend; they are disabled in standalone mode.
+   */
   imageData: string
   posX: number
   posY: number