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/zh.json

@@ -1343,6 +1343,7 @@
     "wallets_created": "创建于",
     "wallets_creating": "正在创建钱包——节点将重启…",
     "wallets_current": "当前",
+    "wallets_empty_hint": "扫描文件夹以查找更多钱包",
     "wallets_exists": "已存在同名钱包。",
     "wallets_external_tt": "在数据目录之外 —「打开」会就地链接（不复制）。",
     "wallets_folder_hint": "/含 .dat 钱包文件的文件夹路径",