feat(combos): add per-combo round-robin strategy

Add ability to configure round-robin strategy for individual combos,
similar to per-provider strategy overrides.

Changes:
- Add comboStrategies setting to store per-combo strategy overrides
- Add Round Robin toggle to each combo card in combos page
- Update chat handler to check combo-specific strategy before global
- Combo-specific strategy takes precedence over global comboStrategy

When enabled, each request to that combo will cycle through providers
instead of always starting with the first one.

Made-with: Cursor

src/lib/localDb.js

@@ -55,6 +55,7 @@ const defaultData = {
     stickyRoundRobinLimit: 3,
     providerStrategies: {},
     comboStrategy: "fallback",
+    comboStrategies: {},
     requireLogin: true,
     observabilityEnabled: true,
     observabilityMaxRecords: 1000,
@@ -83,6 +84,8 @@ function cloneDefaultData() {
       tunnelUrl: "",
       stickyRoundRobinLimit: 3,
       providerStrategies: {},
+      comboStrategy: "fallback",
+      comboStrategies: {},
       requireLogin: true,
       observabilityEnabled: true,
       observabilityMaxRecords: 1000,