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/ipc/settings-handler.ts

@@ -28,7 +28,13 @@ export function registerSettingsHandlers(): void {
 
   ipcMain.handle(IPC_CHANNELS.SETTINGS_GET_USER_TYPE, async (): Promise<IpcResult<UserType>> => {
     return withErrorHandling(
-      async () => (sessionManager.getUserType() as UserType) || 'Guest',
+      async () => {
+        const userType = sessionManager.getUserType()
+        if (!userType) {
+          throw new ValidationError('未找到用户类型', 'VAL_INVALID_INPUT')
+        }
+        return userType as UserType
+      },
       'settings:getUserType'
     )
   })