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>

public/js/config/constants.js

@@ -67,7 +67,27 @@ window.AppConstants.VALIDATION = {
 
     // Max wait threshold (1 - 30 minutes)
     MAX_WAIT_MIN: 60000,
-    MAX_WAIT_MAX: 1800000
+    MAX_WAIT_MAX: 1800000,
+
+    // Rate limit dedup window (1 - 30 seconds)
+    RATE_LIMIT_DEDUP_MIN: 1000,
+    RATE_LIMIT_DEDUP_MAX: 30000,
+
+    // Consecutive failures (1 - 10)
+    MAX_CONSECUTIVE_FAILURES_MIN: 1,
+    MAX_CONSECUTIVE_FAILURES_MAX: 10,
+
+    // Extended cooldown (10 seconds - 5 minutes)
+    EXTENDED_COOLDOWN_MIN: 10000,
+    EXTENDED_COOLDOWN_MAX: 300000,
+
+    // Capacity retry delay (500ms - 10 seconds)
+    CAPACITY_RETRY_DELAY_MIN: 500,
+    CAPACITY_RETRY_DELAY_MAX: 10000,
+
+    // Capacity retries (1 - 10)
+    MAX_CAPACITY_RETRIES_MIN: 1,
+    MAX_CAPACITY_RETRIES_MAX: 10
 };
 
 /**