Interface: hidden views stay mounted and keep using resources

Hidden views still run their widgets. On a machine with several views (video, map, HUD), everything stays in memory even though only one view is on screen, so constrained machines struggle to keep multiple views.

Reproduce: open a profile with more than one view (the ROV default has Video, Map, and HUD). Switch away from a view that has a video player or a map. Those widgets stay in the page and keep their connections and render loops.

`viewsToShow` in `src/stores/widgetManager.ts` returns every view with `visible: true`, and `WidgetsView.vue` mounts all of them stacked with `position: absolute`. There is no setting to mount only the current view.