perf(startup): prepare first-run node files in the background

On a first run or after an upgrade, Windows release builds wrote the
bundled Sapling params, asmap, node and miner (about 88 MB) in
App::init, before the first frame: 0.5-3 s of frozen blank window, 5-30 s
on a slow disk or under antivirus. Zip, AppImage and .app builds copied
the bundled files into the daemon dir and hashed the 51.5 MB of params on
the UI thread at the first node start, a 0.3-1.5 s freeze in the wizard
or the connect loop.

A stat-only check at startup decides whether anything is missing, stale
or unverified; a normal launch starts no thread and shows nothing.
Otherwise one background job extracts (Windows) or copies (Linux, macOS)
the files, then verifies the params once:

- The first frame paints at once. If the job takes longer than 0.4 s, a
  chip at the bottom shows "Preparing wallet files... N%", then
  "Verifying wallet files...".
- A node start on the UI thread while the job runs is deferred and
  replayed when it ends (never into a bootstrap download, a restart or a
  live connection); worker threads wait, and give up on quit. After an
  upgrade that ships a new miner, the node starts after the miner is
  written instead of before. Pool-miner starts and the miner version
  check wait too, so the old miner is never run or reported while it is
  being replaced.
- Each file goes to "<name>.partial", is flushed and renamed into place
  under one lock shared with the daemon start, the bundled-node
  reinstall and the miner extraction, and the job stops between 4 MB
  chunks at shutdown. An interrupted write used to leave a truncated
  binary that was trusted forever; a leftover .partial is now removed.

Other behaviour changes:

- Linux and macOS full-node builds copy the bundled node files (about
  77 MB) into the daemon dir at first launch, even if the wallet ends up
  on an external node; before, this happened at the first embedded start.
- Lite no longer writes the 53 MB of params and asmap it never reads.
- A disk-full or permission failure that leaves the node files unusable
  shows a warning; a miner blocked by antivirus alone does not.
- A corrupt or truncated param in a zip, AppImage or .app install is
  replaced from the bundle and verified again instead of trusted forever.
- Reinstalling the bundled node holds off the connect loop and a deferred
  start for its whole run (either could launch the old binary while the
  new one was written), and is refused while another restart is running.

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

src/util/i18n.cpp

@@ -1513,6 +1513,8 @@ void I18n::loadBuiltinEnglish()
             "Close the program using it (or free the port), then restart — the wallet can't start "
             "its own node while the port is taken.";
         strings_["sb_extracting_sapling"] = "Extracting Sapling parameters...";
+        strings_["startup_files_preparing"] = "Preparing wallet files... %d%%";
+        strings_["startup_files_verifying"] = "Verifying wallet files...";
         strings_["sb_sapling_failed"] = "Failed to extract Sapling parameters.";
         strings_["sb_sapling_not_found"] = "Sapling parameters not found.";
         strings_["sb_daemon_extract_failed"] = "Failed to write daemon files — check free disk space and permissions.";