fix(settings): inject channel_monitor & available_channels into SSR payload

Root cause: GetPublicSettingsForInjection used an inline struct that silently
drifted from dto.PublicSettings and omitted channel_monitor_enabled /
available_channels_enabled. On refresh window.__APP_CONFIG__ lacked these
keys, so cachedPublicSettings.available_channels_enabled resolved to
undefined and the opt-in sidebar entry (=== true) disappeared.

Backend: extract PublicSettingsInjectionPayload as a named type with all
feature-flag fields wired, and add a reflect-based drift test in the dto
package so forgetting a future flag fails CI instead of the browser.

Frontend: introduce utils/featureFlags.ts as the single registry for
public-settings-driven toggles, with explicit opt-in / opt-out modes that
encode the pre-load fallback. AppSidebar switches to makeSidebarFlag() so
adding a new switch only touches the registry.

backend/internal/handler/dto/public_settings_injection_schema_test.go

@@ -0,0 +1,68 @@
+package dto
+
+import (
+	"reflect"
+	"strings"
+	"testing"
+
+	"github.com/Wei-Shaw/sub2api/internal/service"
+)
+
+// TestPublicSettingsInjectionPayload_SchemaDoesNotDrift guarantees the SSR
+// injection struct exposes every JSON field consumed by the frontend.
+//
+// Why this test exists: before we extracted a named PublicSettingsInjectionPayload
+// type, the inline struct was manually kept in sync with dto.PublicSettings and
+// drifted — ChannelMonitorEnabled / AvailableChannelsEnabled were missing, which
+// made the frontend read `undefined` on refresh and hide the "可用渠道" menu
+// until the async /api/v1/settings/public round-trip finished.
+//
+// This test compares the two JSON-tag sets and fails if injection is missing
+// any field that dto.PublicSettings exposes. Adding a new feature flag with
+// only a DTO entry will fail this test until the injection struct is updated.
+//
+// Intentional exclusions (fields present on dto.PublicSettings that SSR does
+// not need to inject) are listed in `dtoOnlyFields` below with a reason.
+func TestPublicSettingsInjectionPayload_SchemaDoesNotDrift(t *testing.T) {
+	injection := jsonTags(reflect.TypeOf(service.PublicSettingsInjectionPayload{}))
+	dtoKeys := jsonTags(reflect.TypeOf(PublicSettings{}))
+
+	// Fields that legitimately live only on the DTO. Keep tiny; document each.
+	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",
+	}
+
+	var missing []string
+	for key := range dtoKeys {
+		if _, ok := injection[key]; ok {
+			continue
+		}
+		if _, allowed := dtoOnlyFields[key]; allowed {
+			continue
+		}
+		missing = append(missing, key)
+	}
+	if len(missing) > 0 {
+		t.Fatalf("service.PublicSettingsInjectionPayload is missing JSON fields present on dto.PublicSettings: %s\n"+
+			"add the field to PublicSettingsInjectionPayload (and GetPublicSettingsForInjection), or "+
+			"document the exclusion in dtoOnlyFields with a reason.", strings.Join(missing, ", "))
+	}
+}
+
+func jsonTags(t reflect.Type) map[string]struct{} {
+	out := make(map[string]struct{})
+	for i := 0; i < t.NumField(); i++ {
+		f := t.Field(i)
+		tag := f.Tag.Get("json")
+		if tag == "" || tag == "-" {
+			continue
+		}
+		name := strings.SplitN(tag, ",", 2)[0]
+		if name == "" {
+			continue
+		}
+		out[name] = struct{}{}
+	}
+	return out
+}