• add EventEmittingButton "save" button (hard-wired label) to Header component
• add Header component to App
• wire up "save" button with the store's save action

While working on this we discussed the question of "header vs application/error/loading state" with @Charlie_WMDE and concluded that the header should be shown irrespective of application state (i.e. will not be covered by either state indicator). The "save" button will need to be disabled during loading/... but this feature will be handled in dedicated T230342: enable/disable save button depending on Bridge state.