Add BOM cost tracking

Suggested-default cost on CableType/DeviceTemplate, copied onto
placed Device instances (same pattern as ports) and overridable
per-connection/per-device in the diagram. BOM view shows per-line
and grand-total cost, excluding (and flagging) anything without a
computable cost rather than treating it as zero.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017DUU6CnxECCDeqDNYJgr5x

src/state/projectStore.ts

@@ -28,7 +28,10 @@ interface ProjectStoreState {
 
   addDeviceFromTemplate: (templateId: string, position: { x: number; y: number }) => void
   updateDevicePosition: (id: string, position: { x: number; y: number }) => void
-  updateDevice: (id: string, patch: Partial<Pick<Device, 'name' | 'category' | 'manufacturer' | 'model' | 'notes'>>) => void
+  updateDevice: (
+    id: string,
+    patch: Partial<Pick<Device, 'name' | 'category' | 'manufacturer' | 'model' | 'notes' | 'cost'>>,
+  ) => void
   removeDevice: (id: string) => void
 
   /** Returns the new port's id, so the caller can move keyboard focus to it. */
@@ -46,7 +49,10 @@ interface ProjectStoreState {
     targetPortId: string
     cableTypeId: string
   }) => boolean
-  updateConnection: (id: string, patch: Partial<Pick<Connection, 'cableTypeId' | 'distance' | 'label' | 'notes'>>) => void
+  updateConnection: (
+    id: string,
+    patch: Partial<Pick<Connection, 'cableTypeId' | 'distance' | 'cost' | 'label' | 'notes'>>,
+  ) => void
   removeConnection: (id: string) => void
 
   addCustomDeviceTemplate: (template: Omit<DeviceTemplate, 'id' | 'builtIn'>) => void