Fixes
- Improved DNS checker behavior in censored VPN mode: DoH providers are now health-probed and auto-fallback is applied when a provider is unreachable.
- Added Auto DoH selection (fastest healthy built-in provider) plus additional DNS providers (AdGuard, Mullvad, OpenDNS).
- Added runtime bridge stability tracking for webtunnel endpoints with temporary suppression of unstable bridge endpoints.
- Added startup stability checks for automatic bridge selection to avoid locking onto unstable bridge types.
- Added tests for DoH settings/provider normalization and custom host/path handling.
- Bumped app + installer versioning to 2.3.1.