fix: stop resetting local usage counters on rate limit window rollover

syncRateLimitWindow() was zeroing request_count, tokens, and
empty_response_count whenever the backend rate limit window changed.
This caused the dashboard to show 0 requests/tokens while the backend
quota still reported usage (e.g. 50% used), because the two data
sources are independent. Local counters now accumulate since account
creation; users can still manually reset via the API.

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

src/auth/account-pool.ts

@@ -286,14 +286,7 @@ export class AccountPool {
 
     const oldResetAt = entry.usage.window_reset_at;
     if (oldResetAt != null && oldResetAt !== newResetAt) {
-      // Window rolled over — reset local counters
-      console.log(`[AccountPool] Rate limit window rolled for ${entryId} (${entry.email ?? "?"}), resetting usage counters`);
-      entry.usage.request_count = 0;
-      entry.usage.input_tokens = 0;
-      entry.usage.output_tokens = 0;
-      entry.usage.empty_response_count = 0;
-      entry.usage.last_used = null;
-      entry.usage.rate_limit_until = null;
+      console.log(`[AccountPool] Rate limit window rolled for ${entryId} (${entry.email ?? "?"}), updating window timestamp`);
     }
     entry.usage.window_reset_at = newResetAt;
     this.schedulePersist();