feat: add "forgot account" reset flow with wallet preservation

Add account reset functionality for users who forgot their login credentials.
The reset clears authentication data while preserving wallet private keys and
exchange configs, which are automatically adopted by the new account on
re-registration to prevent fund loss.

- Add POST /api/reset-account endpoint
- Add "Forgot account?" button on login page (zh/en/id)
- Orphan ai_models and exchanges are re-assigned to new user on register
- Onboarding reuses existing claw402 wallet instead of generating new one

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

store/user.go

@@ -112,6 +112,11 @@ func (s *UserStore) UpdatePassword(userID, passwordHash string) error {
 	}).Error
 }
 
+// DeleteAll deletes all users (reset system to uninitialized state)
+func (s *UserStore) DeleteAll() error {
+	return s.db.Session(&gorm.Session{AllowGlobalUpdate: true}).Delete(&User{}).Error
+}
+
 // EnsureAdmin ensures admin user exists
 func (s *UserStore) EnsureAdmin() error {
 	var count int64