feat: add AI Dental Shopping section with sidebar nav and Login Info page

- Add AI Dental Shopping to sidebar with Search/Tag and Login Info sub-pages
- Build full-stack Login Info CRUD: save vendor name, website, username, password per user
- Add ShoppingVendor Prisma model, run db push, regenerate client and Zod schemas
- Add storage layer, REST API at /api/shopping-vendors/, and frontend table with add/edit/delete modal

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

packages/db/generated/prisma/schema.prisma

@@ -30,6 +30,7 @@ model User {
   npiProviders         NpiProvider[]
   claims               Claim[]
   insuranceCredentials InsuranceCredential[]
+  shoppingVendors      ShoppingVendor[]
   updatedPayments      Payment[]             @relation("PaymentUpdatedBy")
   backups              DatabaseBackup[]
   backupDestinations   BackupDestination[]
@@ -299,6 +300,19 @@ model InsuranceCredential {
   @@index([userId])
 }
 
+model ShoppingVendor {
+  id            Int    @id @default(autoincrement())
+  userId        Int
+  vendorName    String
+  websiteUrl    String
+  loginUsername String
+  loginPassword String
+
+  user User @relation(fields: [userId], references: [id], onDelete: Cascade)
+
+  @@index([userId])
+}
+
 model PdfGroup {
   id        Int         @id @default(autoincrement())
   title     String