feat(user-settings): avatar delete + accent color

- DELETE /users/me/avatar endpoint with file cleanup
- PATCH /users/me/accent-color with hex validation (#rrggbb)
- auth.service.ts: include accentColor in user select
- AccountSettingsForm: delete-avatar button + accent color picker/save/reset
- auth-actions.ts: deleteAvatarAction + updateAccentColorAction

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

apps/api/src/auth/auth.service.ts

@@ -200,6 +200,7 @@ export class AuthService {
         passwordHash: true,
         ldapDn: true,
         avatarPath: true,
+        accentColor: true,
       },
     });