revert: remove fork-only changes from release sync

Revert payment/wechat, sora/claude-max cleanup, fork-only migrations,
and cosmetic changes that were brought in by the release sync commit.
Keep only channel-monitor related improvements:
- PublicSettingsInjectionPayload named struct with drift test
- ChannelMonitorRunner graceful shutdown in wire
- image_output_price in SupportedModelChip
- Simplified buildSelfNavItems in AppSidebar
- Gateway WARN logs for 503 branches

backend/internal/service/setting_service.go

@@ -683,7 +683,6 @@ type PublicSettingsInjectionPayload struct {
 	// Feature flags — MUST match the opt-in/opt-out registry in
 	// frontend/src/utils/featureFlags.ts. Missing a field here is the bug
 	// that hid the "可用渠道" menu on page refresh.
-	ForceEmailOnThirdPartySignup         bool `json:"force_email_on_third_party_signup"`
 	ChannelMonitorEnabled                bool `json:"channel_monitor_enabled"`
 	ChannelMonitorDefaultIntervalSeconds int  `json:"channel_monitor_default_interval_seconds"`
 	AvailableChannelsEnabled             bool `json:"available_channels_enabled"`
@@ -736,7 +735,6 @@ func (s *SettingService) GetPublicSettingsForInjection(ctx context.Context) (any
 		BalanceLowNotifyThreshold:        settings.BalanceLowNotifyThreshold,
 		BalanceLowNotifyRechargeURL:      settings.BalanceLowNotifyRechargeURL,
 
-		ForceEmailOnThirdPartySignup:         settings.ForceEmailOnThirdPartySignup,
 		ChannelMonitorEnabled:                settings.ChannelMonitorEnabled,
 		ChannelMonitorDefaultIntervalSeconds: settings.ChannelMonitorDefaultIntervalSeconds,
 		AvailableChannelsEnabled:             settings.AvailableChannelsEnabled,