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/schemas/designs.py

@@ -0,0 +1,22 @@
+from datetime import datetime
+
+from pydantic import BaseModel
+
+
+class DesignCreate(BaseModel):
+    name: str
+    design_type: str = "electrical"
+
+
+class DesignUpdate(BaseModel):
+    name: str | None = None
+
+
+class DesignResponse(BaseModel):
+    id: str
+    name: str
+    design_type: str
+    created_at: datetime
+    updated_at: datetime
+
+    model_config = {"from_attributes": True}