feat: provider list page availability sort and display

- Add FillProviderAvailability() to batch-fill provider availability data
- Provider list defaults to sort by availability (desc, no-data last)
- Provider cards show availability % with color indicators
- Model rankings default sort back to token usage

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/api.js

@@ -47,6 +47,7 @@ export const getSiteInfo = () => api.get('/api/dist/site/info');
 export const getSiteModels = () => api.get('/api/dist/site/models');
 export const getSitePricing = () => api.get('/api/dist/site/pricing');
 export const getSitePackages = () => api.get('/api/dist/site/packages');
+export const getSiteKeyGroups = () => api.get('/api/dist/site/key-groups');
 
 // ===== Auth =====
 export const register = (data) => api.post('/api/dist/user/register', data);