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/stores/designStore.ts

@@ -0,0 +1,42 @@
+import { create } from 'zustand'
+import type { Design, DesignType } from '@/types'
+
+interface DesignState {
+  designs: Design[]
+  activeDesignId: string | null
+  activeDesignType: DesignType | null
+  loaded: boolean
+  setDesigns: (designs: Design[]) => void
+  setActiveDesign: (id: string) => void
+  getActiveDesign: () => Design | null
+}
+
+export const useDesignStore = create<DesignState>((set, get) => ({
+  designs: [],
+  activeDesignId: null,
+  activeDesignType: null,
+  loaded: false,
+
+  setDesigns: (designs) =>
+    set((state) => {
+      const nextId = state.activeDesignId && designs.find((d) => d.id === state.activeDesignId)
+        ? state.activeDesignId
+        : designs[0]?.id ?? null
+      const nextType = nextId ? designs.find((d) => d.id === nextId)?.design_type ?? null : null
+      return { designs, activeDesignId: nextId, activeDesignType: nextType, loaded: true }
+    }),
+
+  setActiveDesign: (id) =>
+    set((state) => {
+      const design = state.designs.find((d) => d.id === id)
+      return {
+        activeDesignId: id,
+        activeDesignType: design?.design_type ?? null,
+      }
+    }),
+
+  getActiveDesign: () => {
+    const { designs, activeDesignId } = get()
+    return designs.find((d) => d.id === activeDesignId) ?? null
+  },
+}))