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>

tests/run-all.cjs

@@ -9,6 +9,7 @@ const { spawn } = require('child_process');
 const path = require('path');
 
 const tests = [
+    { name: 'Account Selection Strategies', file: 'test-strategies.cjs' },
     { name: 'Thinking Signatures', file: 'test-thinking-signatures.cjs' },
     { name: 'Multi-turn Tools (Non-Streaming)', file: 'test-multiturn-thinking-tools.cjs' },
     { name: 'Multi-turn Tools (Streaming)', file: 'test-multiturn-thinking-tools-streaming.cjs' },