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/handler/dto/public_settings_injection_schema_test.go

@@ -31,6 +31,8 @@ func TestPublicSettingsInjectionPayload_SchemaDoesNotDrift(t *testing.T) {
 	dtoOnlyFields := map[string]string{
 		// sora_client_enabled is an upstream-only field the fork does not surface.
 		"sora_client_enabled": "upstream-only field, not used on this fork",
+		// force_email_on_third_party_signup lives on the DTO but is not injected via SSR.
+		"force_email_on_third_party_signup": "auth-source default, not a feature flag",
 	}
 
 	var missing []string