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/bip-users-dao.ts

@@ -139,7 +139,7 @@ export class BIPUsersDAO {
           return {
             id: row.ID as number,
             username: row.UserName as string,
-            userType: row.UserType as 'Admin' | 'User' | 'Guest'
+            userType: row.UserType as 'Admin' | 'User'
           }
         }
         return null
@@ -157,7 +157,7 @@ export class BIPUsersDAO {
           return {
             id: row.ID as number,
             username: row.UserName as string,
-            userType: row.UserType as 'Admin' | 'User' | 'Guest'
+            userType: row.UserType as 'Admin' | 'User'
           }
         }
         return null
@@ -198,7 +198,7 @@ export class BIPUsersDAO {
           return {
             id: row.ID as number,
             username: row.UserName as string,
-            userType: row.UserType as 'Admin' | 'User' | 'Guest'
+            userType: row.UserType as 'Admin' | 'User'
           }
         }
         return null
@@ -216,7 +216,7 @@ export class BIPUsersDAO {
           return {
             id: row.ID as number,
             username: row.UserName as string,
-            userType: row.UserType as 'Admin' | 'User' | 'Guest'
+            userType: row.UserType as 'Admin' | 'User'
           }
         }
         return null
@@ -254,7 +254,7 @@ export class BIPUsersDAO {
       return result.rows.map((row) => ({
         id: row.ID as number,
         username: row.UserName as string,
-        userType: row.UserType as 'Admin' | 'User' | 'Guest',
+        userType: row.UserType as 'Admin' | 'User',
         createTime: row.CreateTime as Date | undefined
       }))
     } catch (error) {
@@ -270,7 +270,7 @@ export class BIPUsersDAO {
    * Create a new user
    * @param username - The username (must be unique)
    * @param password - The password
-   * @param userType - User type ('Admin', 'User', or 'Guest')
+   * @param userType - User type ('Admin' or 'User')
    * @param computerName - Optional computer name for silent login
    * @returns True if successful
    */