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/domain/project.ts

@@ -172,6 +172,7 @@ export function createDeviceFromTemplate(
     manufacturer: template.manufacturer,
     model: template.model,
     ports: template.ports.map((port) => ({ ...port, id: uuid() })),
+    cost: template.cost,
     position,
   }
 }