Sticky routing: dashboard toggle in the Multi Objective feature flags

Part of #393 (sticky routing). **Status: implemented** in #424, pending review/merge.

> [!NOTE]
> Re-scoped: sticky config moved off the routing algorithm — it is the merchant-level `sticky_routing_enabled` feature flag (#414). The dashboard control is therefore a feature-flag card, not a rule-editor toggle.

The switch lives in the **Multi Objective tab → Feature Flags** list beside Cost savings and Volume contracts, wired to `/merchant-account/{id}/features/sticky-routing`. Verified live: enabling the card flips the flag the decide/feedback paths gate on, and the card shows the Enabled badge with a Disable action.