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/api/client.ts

@@ -28,12 +28,16 @@ export const authApi = {
 }
 
 export const canvasApi = {
-  load: () => api.get('/canvas'),
+  load: (design_id?: string) => {
+    const params = design_id ? { design_id } : {}
+    return api.get('/canvas', { params })
+  },
   save: (payload: {
     nodes: object[]
     edges: object[]
     viewport: object
     custom_style?: object | null
+    design_id?: string | null
   }) => api.post('/canvas/save', payload),
 }
 
@@ -89,6 +93,15 @@ export const settingsApi = {
   save: (data: { interval_seconds: number }) => api.post<{ interval_seconds: number }>('/settings', data),
 }
 
+export const designsApi = {
+  list: () => api.get<import('@/types').Design[]>('/designs'),
+  create: (data: { name: string; design_type: string }) =>
+    api.post<import('@/types').Design>('/designs', data),
+  update: (id: string, data: { name?: string }) =>
+    api.put<import('@/types').Design>(`/designs/${id}`, data),
+  delete: (id: string) => api.delete(`/designs/${id}`),
+}
+
 export const zigbeeApi = {
   testConnection: (data: {
     mqtt_host: string