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/materials.ts

@@ -47,7 +47,10 @@ export async function saveCadPartMaterials(
   itemId: string,
   parts: Array<{ part_name: string; material: string }>,
 ) {
-  const res = await api.put(`/orders/${orderId}/items/${itemId}/cad-materials`, { parts })
+  const res = await api.put<{ parts: Array<{ part_name: string; material: string }> }>(
+    `/orders/${orderId}/items/${itemId}/cad-materials`,
+    { parts },
+  )
   return res.data
 }