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/components.tsx

@@ -19,6 +19,7 @@ import {
   maskToLetters,
   PreviewCard,
 } from "./api";
+import { useBackButton } from "./backButton";
 
 // --- Icons ------------------------------------------------------------
 
@@ -148,9 +149,10 @@ export const Icons = {
 /**
  * A centered modal dialog (backdrop + card) shared by [`ConfirmDialog`],
  * [`PromptDialog`], and other dialogs built outside this module (e.g. the
- * command palette). Dismisses on Escape, backdrop click, or the title-bar
- * close button — the explicit button matters most on mobile, where there's
- * no Escape key and a `wide` dialog can leave little to no backdrop to tap.
+ * command palette). Dismisses on Escape, Android back, backdrop click, or the
+ * title-bar close button — the explicit button matters most on mobile, where
+ * there's no Escape key and a `wide` dialog can leave little to no backdrop
+ * to tap.
  */
 export function DialogFrame({
   title,
@@ -169,6 +171,7 @@ export function DialogFrame({
     window.addEventListener("keydown", onKey);
     return () => window.removeEventListener("keydown", onKey);
   }, [onCancel]);
+  useBackButton(onCancel);
 
   return (
     <div className="dialog-backdrop" onClick={onCancel}>
@@ -732,6 +735,7 @@ export function HoverPreviewProvider({ children }: { children: ReactNode }) {
     window.addEventListener("keydown", onKey);
     return () => window.removeEventListener("keydown", onKey);
   }, [clicked, close]);
+  useBackButton(close, !!clicked);
 
   return (
     <HoverCtx.Provider value={{ enter, move, leave }}>