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.h

@@ -40,6 +40,21 @@ PaymentURI parsePaymentURI(const std::string& uri);
  */
 std::string urlDecode(const std::string& encoded);
 
+/**
+ * @brief URL-encode a string (RFC 3986 unreserved kept; ' ' -> %20; others -> %HH).
+ */
+std::string urlEncode(const std::string& value);
+
+/**
+ * @brief Build a canonical DragonX payment URI: drgx:<address>[?amount=..&label=..&memo=..],
+ *        with query parameters URL-encoded. amount<=0 and empty label/memo are omitted.
+ *
+ * Single source of truth so the receive tab and the request-payment dialog can't diverge on the
+ * scheme or encoding (the receive tab previously emitted an unparseable "dragonx:" URI with a raw memo).
+ */
+std::string buildPaymentUri(const std::string& address, double amount = 0.0,
+                            const std::string& label = "", const std::string& memo = "");
+
 /**
  * @brief Check if a string is a valid payment URI
  * @param str String to check