chore(lint): declare contract callbacks property-style

unbound-method flags destructuring a method-style member (method
signatures are bivariant and exempt from the this-context check).
These contract members are all plain callbacks — declare them as
property-style function types so consumers can destructure them
without a false this-binding hazard. Type-level only.

packages/client/ui-slash/src/client/slots.ts

@@ -34,5 +34,5 @@ export interface MenuViewInjected {
    * @param source - source (group) name.
    * @param index - candidate index within the group.
    */
-  onPick(source: string, index: number): void
+  onPick: (source: string, index: number) => void
 }