Nezha: OAuth2 redirect_uri Host header injection regression when dashboard_host is empty

## Summary Nezha v2.2.3 regresses the GHSA-9rc6-8cjv-rcvx host header injection fix for deployments where the new `dashboard_host` setting is empty. In that configuration, `/api/v1/oauth2/{provider}` again reflects the request `Host` header into the OAuth2 `redirect_uri` sent to the identity provider, even if `install_host` is configured. ## Impact An attacker who can get a victim to start OAuth2 login through a request that reaches Nezha with a forged `Host` header can make Nezha send an attacker-controlled callback URL as the OAuth2 `redirect_uri`. If the configured OAuth2 provider accepts that redirect URI, the victim's authorization code is sent to the attacker origin. The attacker can then complete the OAuth2 binding or login flow described in GHSA-9rc6. This is configuration-dependent, but it affects a realistic upgrade/default state because `dashboard_host` is a new optional field. The original v2.2.0 fix fell back to `install_host`; current v2.2.3 only falls back when `dashboard_host` is non-empty. ## Reproduction On current v2.2.3 / master commit `3d74cd9431a48fa89c6489689eac82a872799ea0`, configure OAuth2 and set an existing dashboard host only through `install_host`, leaving `dashboard_host` empty. Then send the OAuth2 redirect request with a forged Host header: ```http GET /api/v1/oauth2/github?type=1 HTTP/1.1 Host: evil.attacker.test X-Forwarded-Proto: https ``` The generated OAuth2 authorization URL contains: ```text redirect_uri=https%3A%2F%2Fevil.attacker.test