feat(channels): themed model popover + group-badge with rate, subscription & exclusivity

Pricing popover:
- Teleport to body + fixed-position re-measuring on hover/scroll/resize so it
  escapes the card's overflow-hidden clip that was chopping off the lower
  half of the panel.
- Header + border adopt the platform theme via platformBorderClass /
  platformBadgeLightClass so each model card reads at a glance.

Accessible groups:
- Backend AvailableGroupRef / user DTO now expose subscription_type,
  rate_multiplier and is_exclusive. User-specific rates continue to come
  from /groups/rates (same pattern as the API keys page).
- Table renders groups through the shared GroupBadge, which already deepens
  subscription-type colors and shows default vs custom rate as
  <s>default</s> <b>custom</b>.
- Exclusive groups are labelled with a purple shield row, public groups
  with a grey globe row so admins and users can tell at a glance which
  groups they got via explicit grant vs. public access.

i18n keys for exclusive / public / their tooltips added to zh + en.

backend/internal/service/channel_available.go

@@ -8,10 +8,17 @@ import (
 )
 
 // AvailableGroupRef 渠道视图中关联分组的简要信息。
+//
+// 用户侧「可用渠道」页面据此展示：专属分组 vs 公开分组（IsExclusive）、
+// 订阅 vs 标准（SubscriptionType）、默认倍率（RateMultiplier）。用户专属倍率
+// 不在这里暴露，前端自己通过 /groups/rates 拉取，和 API 密钥页面保持一致。
 type AvailableGroupRef struct {
-	ID       int64
-	Name     string
-	Platform string
+	ID               int64
+	Name             string
+	Platform         string
+	SubscriptionType string
+	RateMultiplier   float64
+	IsExclusive      bool
 }
 
 // AvailableChannel 可用渠道视图：用于「可用渠道」页面展示渠道基础信息 +
@@ -49,9 +56,12 @@ func (s *ChannelService) ListAvailable(ctx context.Context) ([]AvailableChannel,
 	for i := range groups {
 		g := groups[i]
 		groupByID[g.ID] = AvailableGroupRef{
-			ID:       g.ID,
-			Name:     g.Name,
-			Platform: g.Platform,
+			ID:               g.ID,
+			Name:             g.Name,
+			Platform:         g.Platform,
+			SubscriptionType: g.SubscriptionType,
+			RateMultiplier:   g.RateMultiplier,
+			IsExclusive:      g.IsExclusive,
 		}
 	}