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

.env.example

@@ -1,6 +1,9 @@
 # Backend - server-side only (NEVER commit .env)
 SECRET_KEY=change_me_in_production
 SQLITE_PATH=./data/homelab.db
+# Uploaded media (floor plans) folder. Optional — defaults to <SQLITE_PATH dir>/uploads,
+# which sits on the same persistent volume as the DB.
+# UPLOAD_DIR=./data/uploads
 # Set this to the URL(s) you use to access Homelable in your browser.
 CORS_ORIGINS=["http://localhost:5173","http://localhost:3000"]