refactor: replace scope-level rate limiting with model-level rate limiting

Merge functional changes from develop branch:
- Remove AntigravityQuotaScope system (claude/gemini_text/gemini_image)
- Replace with per-model rate limiting using resolveAntigravityModelKey
- Remove model load statistics (IncrModelCallCount/GetModelLoadBatch)
- Simplify account selection to unified priority→load→LRU algorithm
- Remove SetAntigravityQuotaScopeLimit from AccountRepository
- Clean up scope-related UI indicators and API fields

frontend/src/i18n/locales/en.ts

@@ -1360,7 +1360,6 @@ export default {
         overloaded: 'Overloaded',
         tempUnschedulable: 'Temp Unschedulable',
         rateLimitedUntil: 'Rate limited until {time}',
-        scopeRateLimitedUntil: '{scope} rate limited until {time}',
         modelRateLimitedUntil: '{model} rate limited until {time}',
         overloadedUntil: 'Overloaded until {time}',
         viewTempUnschedDetails: 'View temp unschedulable details'
@@ -3063,7 +3062,6 @@ export default {
         empty: 'No data',
         queued: 'Queue {count}',
         rateLimited: 'Rate-limited {count}',
-        scopeRateLimitedTooltip: '{scope} rate-limited ({count} accounts)',
         errorAccounts: 'Errors {count}',
         loadFailed: 'Failed to load concurrency data'
       },