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>

src/util/i18n.cpp

@@ -383,6 +383,7 @@ void I18n::loadBuiltinEnglish()
         strings_["wallets_external_tt"] = "Outside your data directory \xE2\x80\x94 Open links it in place (no copy).";
         strings_["wallets_scan_folder"] = "Scan another folder for wallets\xE2\x80\xA6";
         strings_["wallets_scanned_folders"] = "Scanned folders:";
+        strings_["wallets_scanning"] = "Scanning folders\xE2\x80\xA6";
         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.";