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.

frontend/src/api/channels.ts

@@ -10,6 +10,12 @@ export interface UserAvailableGroup {
   id: number
   name: string
   platform: string
+  /** 'standard' | 'subscription' — 订阅分组视觉加深，和 API 密钥页保持一致。 */
+  subscription_type: string
+  /** 分组默认倍率。用户专属倍率（若有）通过 /groups/rates 获取后在前端 join。 */
+  rate_multiplier: number
+  /** true = 专属分组（小范围授权）；false = 公开分组。 */
+  is_exclusive: boolean
 }
 
 export interface UserPricingInterval {