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/translations/en.js

@@ -237,6 +237,18 @@ window.translations.en = {
     defaultCooldownDesc: "Fallback cooldown when API doesn't provide a reset time.",
     maxWaitThreshold: "Max Wait Before Error",
     maxWaitDesc: "If all accounts are rate-limited longer than this, error immediately instead of waiting.",
+    // Error Handling Tuning
+    errorHandlingTuning: "Error Handling Tuning",
+    rateLimitDedupWindow: "Rate Limit Dedup Window",
+    rateLimitDedupWindowDesc: "Prevents concurrent retry storms when multiple requests hit rate limits simultaneously.",
+    maxConsecutiveFailures: "Max Consecutive Failures",
+    maxConsecutiveFailuresDesc: "Number of consecutive failures before applying extended cooldown to an account.",
+    extendedCooldown: "Extended Cooldown",
+    extendedCooldownDesc: "Cooldown duration applied after max consecutive failures reached.",
+    capacityRetryDelay: "Capacity Retry Delay",
+    capacityRetryDelayDesc: "Delay before retrying when model capacity is exhausted (not quota).",
+    maxCapacityRetries: "Max Capacity Retries",
+    maxCapacityRetriesDesc: "Maximum retries for capacity exhaustion before switching accounts.",
     saveConfigServer: "Save Configuration",
     serverRestartAlert: "Changes saved to {path}. Restart server to apply some settings.",
     changePassword: "Change WebUI Password",
@@ -318,6 +330,18 @@ window.translations.en = {
     failedToUpdateModelConfig: "Failed to update model config",
     fieldUpdated: "{displayName} updated to {value}",
     failedToUpdateField: "Failed to update {displayName}",
+    // Account Selection Strategy
+    accountSelectionStrategy: "Account Selection Strategy",
+    selectionStrategy: "Selection Strategy",
+    strategyStickyLabel: "Sticky (Cache Optimized)",
+    strategyRoundRobinLabel: "Round Robin (Load Balanced)",
+    strategyHybridLabel: "Hybrid (Smart Distribution)",
+    strategyStickyDesc: "Stays on same account until rate-limited. Best for prompt caching.",
+    strategyRoundRobinDesc: "Rotates to next account on every request. Maximum throughput.",
+    strategyHybridDesc: "Smart selection based on health, tokens, and freshness.",
+    strategyUpdated: "Strategy updated to: {strategy}",
+    failedToUpdateStrategy: "Failed to update strategy",
+    invalidStrategy: "Invalid strategy selected",
     // Validation Messages
     mustBeValidNumber: "{fieldName} must be a valid number",
     mustBeAtLeast: "{fieldName} must be at least {min}",