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.

frontend/src/components/modals/NodeModal.tsx

@@ -19,6 +19,7 @@ const NODE_TYPE_GROUPS: { label: string; types: NodeType[] }[] = [
   { label: 'IoT',            types: ['iot', 'camera', 'cpl'] },
   { label: 'Zigbee',         types: ['zigbee_coordinator', 'zigbee_router', 'zigbee_enddevice'] },
   { label: 'Personal',       types: ['computer', 'laptop', 'mobile'] },
+  { label: 'Electrical',     types: ['grid', 'ups', 'battery', 'generator', 'solar_panel', 'inverter', 'circuit_breaker', 'contactor', 'electrical_switch', 'socket', 'light', 'meter', 'transformer', 'load'] },
   { label: 'Generic',        types: ['generic', 'groupRect'] },
 ]