Add combo round-robin strategy to distribute load across providers (#390)

- Add comboRotationState Map to track rotation per combo
- Add getRotatedModels() to rotate model order based on strategy
- Pass comboName and comboStrategy to handleComboChat()
- Add comboStrategy setting (default: fallback)
- Add UI toggle for Combo Round Robin in profile settings

When enabled, each request to a combo starts with a different provider
instead of always starting with the first one, distributing load evenly.

Co-authored-by: Antigravity Agent <antigravity@example.com>

src/lib/localDb.js

@@ -54,6 +54,7 @@ const defaultData = {
     tunnelUrl: "",
     stickyRoundRobinLimit: 3,
     providerStrategies: {},
+    comboStrategy: "fallback",
     requireLogin: true,
     observabilityEnabled: true,
     observabilityMaxRecords: 1000,