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

backend/app/main.py

@@ -7,7 +7,7 @@ from typing import Any
 from fastapi import FastAPI
 from fastapi.middleware.cors import CORSMiddleware
 
-from app.api.routes import auth, canvas, designs, edges, liveview, nodes, scan, stats, status, zigbee, zwave
+from app.api.routes import auth, canvas, designs, edges, liveview, media, nodes, scan, stats, status, zigbee, zwave
 from app.api.routes import settings as settings_routes
 from app.core.config import settings
 from app.core.scheduler import start_scheduler, stop_scheduler
@@ -59,6 +59,7 @@ app.include_router(liveview.router, prefix="/api/v1/liveview", tags=["liveview"]
 app.include_router(zigbee.router, prefix="/api/v1/zigbee", tags=["zigbee"])
 app.include_router(zwave.router, prefix="/api/v1/zwave", tags=["zwave"])
 app.include_router(stats.router, prefix="/api/v1/stats", tags=["stats"])
+app.include_router(media.router, prefix="/api/v1/media", tags=["media"])
 
 
 @app.get("/api/v1/health")