refactor(section-d): frontend API client type safety audit

All 19 api/*.ts files already used import api from './client' (X-Tenant-ID
guaranteed). Fixed missing type generics and any usage in 10 files:
- worker.ts: args: any[] → unknown[]
- imports.ts, notifications.ts: add response type generics
- renderTemplates.ts: add typed generics on 6 calls
- materials.ts, cad.ts: type previously untyped api calls
- products.ts: ProductCadUploadResponse interface, typed generics
- uploads.ts: StepUploadResponse interface
- billing.ts, orders.ts: <Blob> on download calls

Zero bare axios usage, zero as any in API layer.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/api/imports.ts

@@ -14,7 +14,7 @@ export interface ImportValidation {
 }
 
 export async function getImportValidation(id: string): Promise<ImportValidation> {
-  const res = await api.get(`/imports/validation/${id}`)
+  const res = await api.get<ImportValidation>(`/imports/validation/${id}`)
   return res.data
 }