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>

src/util/i18n.cpp

@@ -289,6 +289,7 @@ void I18n::loadBuiltinEnglish()
         strings_["wallets_scan_folder"] = "Scan another folder for wallets\xE2\x80\xA6";
         strings_["wallets_scanned_folders"] = "Scanned folders:";
         strings_["wallets_remove_folder"] = "Stop scanning this folder";
+        strings_["wallets_empty_hint"] = "Scan a folder to find more wallets";
         strings_["wallets_folder_invalid"] = "That folder doesn't exist.";
         strings_["wallets_new_label"] = "Create a new wallet:";
         strings_["wallets_new_hint"] = "Name (e.g. savings)";