refactor(auth): remove Guest role from user type system

Remove all references to 'Guest' user type from the codebase, simplifying the role system to only support 'Admin' and 'User' roles.

Changes:
- Update type definitions to exclude 'Guest' from UserType
- Remove isGuest() method from SessionManager
- Remove Guest-specific logic from update services
- Update all type assertions from 'Admin | User | Guest' to 'Admin | User'
- Remove Guest UI styling from UserSelectionDialog
- Replace Guest fallback with ValidationError in settings handler

Error handling:
- Zod schema now rejects 'Guest' as invalid user type
- TypeScript will fail compilation if 'Guest' is referenced
- Runtime errors occur if database contains Guest users

No database migration needed (confirmed: no Guest users exist)

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

src/main/services/update/update-service.ts

@@ -106,7 +106,7 @@ export class UpdateService {
     this.ensureInitialized()
     this.status.currentUserType = userType
 
-    if (!this.status.enabled || !userType || userType === 'Guest') {
+    if (!this.status.enabled || !userType) {
       this.clearPolling()
       this.catalog = { stable: [], preview: [] }
       this.publishStatus({