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>

src/webui/index.js

@@ -282,7 +282,7 @@ export function mountWebUI(app, dirname, accountManager) {
      */
     app.post('/api/config', (req, res) => {
         try {
-            const { debug, logLevel, maxRetries, retryBaseMs, retryMaxMs, persistTokenCache, defaultCooldownMs, maxWaitBeforeErrorMs } = req.body;
+            const { debug, logLevel, maxRetries, retryBaseMs, retryMaxMs, persistTokenCache, defaultCooldownMs, maxWaitBeforeErrorMs, accountSelection } = req.body;
 
             // Only allow updating specific fields (security)
             const updates = {};
@@ -308,6 +308,16 @@ export function mountWebUI(app, dirname, accountManager) {
             if (typeof maxWaitBeforeErrorMs === 'number' && maxWaitBeforeErrorMs >= 0 && maxWaitBeforeErrorMs <= 600000) {
                 updates.maxWaitBeforeErrorMs = maxWaitBeforeErrorMs;
             }
+            // Account selection strategy validation
+            if (accountSelection && typeof accountSelection === 'object') {
+                const validStrategies = ['sticky', 'round-robin', 'hybrid'];
+                if (accountSelection.strategy && validStrategies.includes(accountSelection.strategy)) {
+                    updates.accountSelection = {
+                        ...(config.accountSelection || {}),
+                        strategy: accountSelection.strategy
+                    };
+                }
+            }
 
             if (Object.keys(updates).length === 0) {
                 return res.status(400).json({