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/types/settings.types.ts

@@ -10,7 +10,7 @@
 /**
  * User type for settings permission control
  */
-export type UserType = 'Admin' | 'User' | 'Guest'
+export type UserType = 'Admin' | 'User'
 
 /**
  * Database type selection

src/main/types/user.types.ts

@@ -5,7 +5,7 @@
 /**
  * User type enumeration
  */
-export type UserType = 'Admin' | 'User' | 'Guest'
+export type UserType = 'Admin' | 'User'
 
 /**
  * User information interface