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/payment_uri.cpp

@@ -5,12 +5,58 @@
 #include "payment_uri.h"
 
 #include <sstream>
+#include <iomanip>
 #include <algorithm>
 #include <cctype>
+#include <cstdio>
 
 namespace dragonx {
 namespace util {
 
+std::string urlEncode(const std::string& value)
+{
+    std::string out;
+    out.reserve(value.size());
+    for (unsigned char c : value) {
+        if (std::isalnum(c) || c == '-' || c == '_' || c == '.' || c == '~') {
+            out += static_cast<char>(c);
+        } else if (c == ' ') {
+            out += "%20";
+        } else {
+            char hex[4];
+            std::snprintf(hex, sizeof(hex), "%%%02X", c);
+            out += hex;
+        }
+    }
+    return out;
+}
+
+std::string buildPaymentUri(const std::string& address, double amount,
+                            const std::string& label, const std::string& memo)
+{
+    if (address.empty()) return "";
+
+    std::ostringstream uri;
+    uri << "drgx:" << address;
+
+    bool hasParams = false;
+    auto addParam = [&](const char* key, const std::string& value) {
+        if (value.empty()) return;
+        uri << (hasParams ? "&" : "?") << key << "=" << value;
+        hasParams = true;
+    };
+
+    if (amount > 0.0) {
+        std::ostringstream amt;
+        amt << std::fixed << std::setprecision(8) << amount;
+        addParam("amount", amt.str());
+    }
+    addParam("label", urlEncode(label));
+    addParam("memo", urlEncode(memo));
+
+    return uri.str();
+}
+
 std::string urlDecode(const std::string& encoded)
 {
     std::string result;