fix: Tier-1 UX robustness — backup/export integrity, verified installs, input validation

From the app-wide robustness audit (calibrated to the import-key dialog). These
are the safety-critical fixes; several could cost users funds:

- Backup/export false success (FUND LOSS): exportAllKeys pre-seeded a header, so
  a keyless result (the usual case when the wallet is encrypted+locked) still
  looked non-empty and backupWallet wrote a private-key-less file and reported
  "Backup saved". Now exportAllKeys returns an exported count; backupWallet and
  the export-all dialog refuse to write / report success on 0 keys, disclose the
  count, flag partial results as INCOMPLETE, and the backup dialog confirms
  before overwriting an existing file (+ trims the path).
- Bootstrap: fail CLOSED — refuse to install an unverified multi-GB archive when
  no checksum is published (was `return true`), mirroring the xmrig/daemon updaters.
- Restore-from-seed: require a valid BIP39 word count (12/15/18/21/24) with a live
  "should be 24 words — you have N" hint instead of accepting any non-empty text.
- Encryption passphrase: detect leading/trailing whitespace and BLOCK with a
  warning (not a silent trim, which would change the passphrase and lock the user
  out).
- Receive payment QR: emit the canonical `drgx:` scheme with a URL-encoded memo
  via a new shared util::buildPaymentUri (the request-payment dialog now routes
  through it too, so they can't diverge); the old "dragonx:" + raw memo was
  unparseable by the wallet's own scanner.

Also clamps the receive "Recent Received" list to the 4 most recent (companion to
the recent-lists commit).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/util/bootstrap.cpp

@@ -724,9 +724,13 @@ bool Bootstrap::verifyChecksums(const std::string& zipPath, const std::string& b
     bool haveMD5    = !md5Content.empty();
 
     if (!haveSHA256 && !haveMD5) {
-        DEBUG_LOGF("[Bootstrap] Warning: no checksum files available — skipping verification\n");
-        // Allow the process to continue (server may not have checksum files yet)
-        return true;
+        // Fail CLOSED: refuse to install an unverified multi-GB archive (mirrors the xmrig/daemon
+        // updaters, which refuse when no checksum is published). Trusting an unverified download of
+        // this size is a real integrity/security risk; a missing checksum is a server-side gap to fix.
+        DEBUG_LOGF("[Bootstrap] No checksum published — refusing to install an unverified archive\n");
+        setProgress(State::Failed, "Verification failed: no checksum was published for the bootstrap. "
+                                   "Refusing to install an unverified archive.");
+        return false;
     }
 
     // Determine progress ranges: if both checksums exist, split 0-50% / 50-100%