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/i18n/locales/en.ts

@@ -938,6 +938,10 @@ export default {
     empty: 'No available channels',
     noModels: 'No models configured',
     noPricing: 'Pricing not configured',
+    exclusive: 'Exclusive',
+    public: 'Public',
+    exclusiveTooltip: 'Exclusive groups granted to you by an admin',
+    publicTooltip: 'Groups open to all users',
     columns: {
       name: 'Channel',
       platform: 'Platform',