perf(wallets): walk added wallet folders off the UI thread

Every open of the Wallets dialog, Refresh, folder add or remove, and Open
on a stale row walked each user-added folder recursively on the UI thread
before the dialog drew: up to 40,000 entries per folder, roughly
50-150 ms warm on Linux and seconds on a cold disk, a network share or
Windows.

The datadir listing (a few top-level files) stays synchronous, so a
wallet just created shows at once. The recursive walk runs on a detached
worker with the same depth, hit and entry limits; its rows are merged at
the start of a later frame and only the new rows are probed.

- The dialog opens at once with the datadir rows. The external rows
  arrive when the walk ends, with "Scanning folders..." beside the
  scanned-folders caption, and the card grows once.
- Reopening within 60 s with the same folders reuses the last walk
  without touching the disk, so its rows and sizes can be up to a minute
  stale: a wallet copied into a folder in that minute needs Refresh.
  Refresh and Open on a stale row always walk again.
- While a walk or a wallet probe runs (up to 30 s) the dialog keeps
  frames coming, so rows and badges appear without moving the mouse.
- The 200-row cap now holds across all folders; each extra folder could
  add one row past it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

res/lang/ja.json

@@ -2304,6 +2304,7 @@
     "wallets_reveal": "フォルダを開く",
     "wallets_scan_folder": "別のフォルダをスキャンしてウォレットを検索…",
     "wallets_scanned_folders": "スキャン対象フォルダー：",
+    "wallets_scanning": "フォルダをスキャンしています…",
     "wallets_sort_addresses": "アドレス",
     "wallets_sort_asc": "昇順（古い/少ない/小さい順）",
     "wallets_sort_by": "並べ替え：",