feat: add configurable account selection strategies

Refactor account selection into a strategy pattern with three options:
- Sticky: cache-optimized, stays on same account until rate-limited
- Round-robin: load-balanced, rotates every request
- Hybrid (default): smart distribution using health scores, token buckets, and LRU

The hybrid strategy uses multiple signals for optimal account selection:
health tracking for reliability, client-side token buckets for rate limiting,
and LRU freshness to prefer rested accounts.

Includes WebUI settings for strategy selection and unit tests.

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

package.json

@@ -34,7 +34,8 @@
     "test:crossmodel": "node tests/test-cross-model-thinking.cjs",
     "test:oauth": "node tests/test-oauth-no-browser.cjs",
     "test:emptyretry": "node tests/test-empty-response-retry.cjs",
-    "test:sanitizer": "node tests/test-schema-sanitizer.cjs"
+    "test:sanitizer": "node tests/test-schema-sanitizer.cjs",
+    "test:strategies": "node tests/test-strategies.cjs"
   },
   "keywords": [
     "claude",