feat: multi-design canvas system with electrical nodes/edges

Backend: New Design model + designs table; design_id FK on nodes, edges, canvas_state; migration seeds default 'Network Topology' design; full CRUD API for designs; canvas load/save accept design_id.

Frontend: designStore (Zustand), design switcher in Sidebar, design-aware canvas load/save, auto-save on design switch.

Electrical node types (14): grid, ups, battery, generator, solar_panel, inverter, circuit_breaker, contactor, electrical_switch, socket, light, meter, transformer, load — icons, registrations, accent colors in all 6 themes.

Electrical edge type: registered in edgeTypes, BASE_STYLES, edgeColors, all theme edgeColors, EDGE_DEFAULT_COLORS.

Bug fixes: data corruption on design switch (stale closure), race condition on save-then-load, missing Zap import, missing Electrical group in NodeModal, missing electrical entries in custom theme edgeColors, inline imports hoisted.

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, edges, liveview, nodes, scan, stats, status, zigbee
+from app.api.routes import auth, canvas, designs, edges, liveview, nodes, scan, stats, status, zigbee
 from app.api.routes import settings as settings_routes
 from app.core.config import settings
 from app.core.scheduler import start_scheduler, stop_scheduler
@@ -51,6 +51,7 @@ app.include_router(auth.router, prefix="/api/v1/auth", tags=["auth"])
 app.include_router(nodes.router, prefix="/api/v1/nodes", tags=["nodes"])
 app.include_router(edges.router, prefix="/api/v1/edges", tags=["edges"])
 app.include_router(canvas.router, prefix="/api/v1/canvas", tags=["canvas"])
+app.include_router(designs.router, prefix="/api/v1/designs", tags=["designs"])
 app.include_router(scan.router, prefix="/api/v1/scan", tags=["scan"])
 app.include_router(status.router, prefix="/api/v1/status", tags=["status"])
 app.include_router(settings_routes.router, prefix="/api/v1/settings", tags=["settings"])