UNO polish: center nav-rail items, drop per-page XP bar, shop category tabs

- NavRail: vertically center items in the side rail (was top-aligned).
- ScreenHeader: showXp defaults off — the level/XP bar no longer clutters every
  sub-page (it lives on Home's chip + the Profile page).
- Shop: category tabs (avatars / fronts / backs / reactions / stickers / titles
  / XP) so only one category shows at a time — no more endless scroll.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/i18n.tsx

@@ -320,6 +320,7 @@ const fa: Dict = {
   "shop.reqLevel": "سطح",
   "shop.reqRating": "امتیاز",
   "shop.reqAchv": "دستاورد:",
+  "shop.emptyCat": "آیتمی در این دسته نیست",
   "reward.newTitle": "عنوان جدید",
 
   "reactions.title": "شکلک",
@@ -666,6 +667,7 @@ const en: Dict = {
   "shop.reqLevel": "Level",
   "shop.reqRating": "Rating",
   "shop.reqAchv": "Achievement:",
+  "shop.emptyCat": "No items in this category",
   "reward.newTitle": "New title",
 
   "reactions.title": "Emoji",