feat: add standalone (frontend-only) mode via VITE_STANDALONE build flag

- App.tsx: bypass auth gate, swap canvas save/load to localStorage
- Sidebar.tsx: hide scan views and Scan Network button
- useStatusPolling.ts: skip WebSocket connection
- Dockerfile.frontend: accept VITE_STANDALONE build arg
- docker-publish.yml: build ghcr.io/pouzor/homelable-frontend-standalone image
- docker-compose.standalone.yml: frontend-only compose file
- install.sh: add --standalone flag

Usage:
  curl -fsSL .../install.sh | bash -s -- --standalone
  cd homelable && docker compose up -d

Dockerfile.frontend

@@ -1,6 +1,9 @@
 # Stage 1: build
 FROM node:20-alpine AS builder
 
+ARG VITE_STANDALONE=false
+ENV VITE_STANDALONE=$VITE_STANDALONE
+
 WORKDIR /app
 COPY frontend/package*.json ./
 RUN npm ci