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/user/session-manager.ts

@@ -126,13 +126,6 @@ export class SessionManager {
     return this.currentUser?.userType === 'Admin'
   }
 
-  /**
-   * Check if the current user is a guest
-   */
-  public isGuest(): boolean {
-    return this.currentUser?.userType === 'Guest'
-  }
-
   /**
    * Get the current username
    */