Android back button closes modals and leaves the News set browser

Adds useBackButton, a stack of back-button claimants (last claimed wins)
backed by Tauri's app-plugin back-button event. The listener is only
registered while something has claimed the button, so at the root back
keeps its native leave-the-app behavior.

Claimed by: the click-to-preview modal, DialogFrame (so Suggest cards,
Sample hand and every other dialog), and NewsView while a set's card
list is open. A preview opened from inside a dialog closes first.

src/views/NewsView.tsx

@@ -9,6 +9,7 @@ import {
   applyCardCriteria,
   useCardCriteria,
 } from "../components";
+import { useBackButton } from "../backButton";
 
 /** Sets whose release date is still ahead of us, newest-announced first. */
 function splitByRelease(sets: SetInfo[]): { upcoming: SetInfo[]; released: SetInfo[] } {
@@ -98,6 +99,10 @@ export function NewsView() {
     load(false);
   }, [load]);
 
+  // Android back from a set's card list returns to the news, like the
+  // "Back to news" button.
+  useBackButton(() => setOpenSet(null), !!openSet);
+
   function openSetBrowser(set: SetInfo) {
     setOpenSet(set);
     setCards(null);