fix(send): accept P2SH/multisig recipients in the send + URI address gates

Same defect class as the import-key fix: a hardcoded prefix/length pre-filter
layered over the checksum validators rejected valid addresses before the daemon
saw them. The send-screen recipient gate required a[0]=='R', and the payment-URI
parser accepted only 'R'/'t' with rigid length bands — so every valid P2SH /
multisig address (DragonX SCRIPT_ADDRESS=85 → 'b…') was silently refused, leaving
the Send button disabled with no usable recipient.

Centralize recipient recognition in util/address_validation:
- isTransparentAddress: Base58Check with a 21-byte version+hash160 payload —
  covers P2PKH ('R…', v60) AND P2SH ('b…', v85) on every network, rejects WIF
  keys / typos by real checksum.
- isShieldedAddress: Bech32 + a Sapling payment-address HRP (zs / ztestsapling /
  zregtestsapling), distinguishing a payment address from a viewing key.
- isValidRecipientAddress: either of the above.

send_tab's two validity helpers (the single choke point for all 5 call sites) and
the payment-URI format check now route through these. The URI parser now
checksum-validates the recipient (fail-fast on transcription errors) rather than
being prefix/length-only.

Tests use real checksummed vectors (P2PKH/P2SH/shielded, WIF- and typo-rejection);
testPaymentUri updated off its old fake fixed-char addresses. Suite green (1/1).

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

src/util/payment_uri.cpp

@@ -3,6 +3,7 @@
 // Released under the GPLv3
 
 #include "payment_uri.h"
+#include "address_validation.h"
 
 #include <sstream>
 #include <iomanip>
@@ -161,20 +162,11 @@ PaymentURI parsePaymentURI(const std::string& uri)
         return result;
     }
     
-    // Basic address format check. NOTE: this is format-only by design — the send flow
-    // checksum-validates the recipient (isValidBase58Check / shielded check) before broadcasting,
-    // so an invalid-checksum address parsed here can never actually be sent to.
-    bool validFormat = false;
-
-    // z-address: starts with 'zs' and is 78+ chars
-    if (result.address[0] == 'z' && result.address.size() >= 78) {
-        validFormat = true;
-    }
-    // t-address: starts with 'R' (DragonX) or 't' (HUSH) and is ~34 chars
-    else if ((result.address[0] == 'R' || result.address[0] == 't') &&
-             result.address.size() >= 26 && result.address.size() <= 36) {
-        validFormat = true;
-    }
+    // Address format check via the shared, structure-based recognizers (checksum + real DragonX
+    // address types). This accepts shielded ("zs…"), P2PKH ("R…") and P2SH/multisig ("b…") forms —
+    // the old prefix/length heuristic rejected P2SH and hardcoded a 't' prefix DragonX never emits.
+    const bool validFormat = isShieldedAddress(result.address) ||
+                             isTransparentAddress(result.address);
 
     if (!validFormat) {
         result.error = "Invalid address format";