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/handler/available_channel_handler_test.go

@@ -101,6 +101,17 @@ func TestUserAvailableChannel_FieldWhitelist(t *testing.T) {
 		require.Truef(t, exists, "platform section must expose %q", key)
 	}
 
+	// Group DTO 暴露区分专属/公开、订阅类型、默认倍率所需的字段，
+	// 前端据此渲染 GroupBadge 并与 API 密钥页保持一致的视觉。
+	rawGroup, err := json.Marshal(row.Platforms[0].Groups[0])
+	require.NoError(t, err)
+	var groupDecoded map[string]any
+	require.NoError(t, json.Unmarshal(rawGroup, &groupDecoded))
+	for _, key := range []string{"id", "name", "platform", "subscription_type", "rate_multiplier", "is_exclusive"} {
+		_, exists := groupDecoded[key]
+		require.Truef(t, exists, "group DTO must expose %q", key)
+	}
+
 	// pricing interval 白名单：不应暴露 id / sort_order。
 	pricing := toUserPricing(&service.ChannelModelPricing{
 		BillingMode: service.BillingModeToken,