fix(ux): apply UX quick wins (#54)

Get started links to registration and says an admin approves accounts.
Back navigation restores the library scroll; search no longer fills history.
Steam and PSN unlink ask first. Any close marks news as seen.
Confetti plays only when the level rises on the page.
Count-ups respect reduced motion. Html lang follows the UI language.
Add a 404 page, translated role and dialog close label.

src/utils/confetti.js

@@ -15,15 +15,15 @@ export const celebrationLevel = (game) => {
 };
 
 /**
- * The confetti to play when the page shows this game: on opening a finished game, and when the level rises on the
- * same page (e.g. the user sets FINISHED). A reload of the same game plays nothing.
+ * The confetti to play: only when the level rises while the page shows the game (the user sets Finished or completes
+ * it here). Opening a finished game and a reload play nothing.
  *
  * @param before {appid, level} of the game shown before, null on opening the page
  * @returns 'small', 'big' or null
  */
 export const celebrationFor = (before, game) => {
+    if (before?.appid !== game.appid) return null;
     const level = celebrationLevel(game);
-    const from = before?.appid === game.appid ? before.level : 0;
-    if (level <= from) return null;
+    if (level <= before.level) return null;
     return level === 2 ? 'big' : 'small';
 };