feat(wallets): subtle empty-state hint in the fixed-height list

With the list now pinned at max height, a few wallets leave blank space below
the cards. Fill it — only when there's real room to spare — with a subtle
centered folder glyph + "Scan a folder to find more wallets" (OnSurfaceDisabled),
so the area reads as a gentle nudge toward the scan action rather than dead
space. Purely decorative (draw-list only, clipped to the list); the actions stay
below. New i18n key wallets_empty_hint, translated across all 8 languages.

Verified on the sweep: the hint centers in the gap and reads subtly on both dark
and light themes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

res/lang/ko.json

@@ -1343,6 +1343,7 @@
     "wallets_created": "생성",
     "wallets_creating": "지갑을 만드는 중 — 노드가 재시작됩니다…",
     "wallets_current": "현재",
+    "wallets_empty_hint": "폴더를 스캔하여 다른 지갑 찾기",
     "wallets_exists": "같은 이름의 지갑이 이미 존재합니다.",
     "wallets_external_tt": "데이터 디렉터리 밖 — '열기'는 제자리에 링크합니다 (복사 없음).",
     "wallets_folder_hint": "wallet-*.dat 파일이 있는 폴더 경로",