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>

tests/test_phase4.cpp

@@ -726,7 +726,9 @@ void testConnectionConfig()
 
 void testPaymentUri()
 {
-    std::string taddr = "R" + std::string(33, 'a');
+    // Real checksummed addresses — the parser now checksum-validates the recipient (not a bare
+    // prefix/length filter), so it accepts P2PKH / P2SH / shielded and rejects transcription errors.
+    std::string taddr = "R9NXAVJezHiBnT3ijTpg3JUZre7PxhJWti";                 // P2PKH (v60)
     auto parsed = dragonx::util::parsePaymentURI(
         "drgx:" + taddr + "?amount=1.25000000&label=Main+Wallet&memo=hello%20there&message=thanks");
 
@@ -737,11 +739,19 @@ void testPaymentUri()
     EXPECT_EQ(parsed.memo, std::string("hello there"));
     EXPECT_EQ(parsed.message, std::string("thanks"));
 
-    std::string zaddr = "zs" + std::string(76, 'b');
+    std::string zaddr = "zs1qqqsyqcyq5rqwzqfpg9scrgwpugpzysnzs23v9ccrydpk8qarc0jqgfzyvjz2f389q5j5ctfvp5";
     auto zparsed = dragonx::util::parsePaymentURI("hush://" + zaddr + "?amt=0.5");
     EXPECT_TRUE(zparsed.valid);
     EXPECT_NEAR(zparsed.amount, 0.5, 0.00000001);
 
+    // Regression: a P2SH/multisig recipient ("b…", v85) must parse — the old 'R'/'t'-only filter dropped it.
+    auto p2sh = dragonx::util::parsePaymentURI("drgx:bCpbnCkrjoJ6EHXtLx9eASHEbFYyikt35C?amount=1");
+    EXPECT_TRUE(p2sh.valid);
+
+    // A transcription error (flipped checksum char) is now rejected at parse time.
+    auto typo = dragonx::util::parsePaymentURI("drgx:R9NXAVJezHiBnT3ijTpg3JUZre7PxhJWtX?amount=1");
+    EXPECT_FALSE(typo.valid);
+
     auto invalid = dragonx::util::parsePaymentURI("drgx:" + taddr + "?amount=-1");
     EXPECT_FALSE(invalid.valid);
     EXPECT_EQ(invalid.error, std::string("Invalid negative amount"));
@@ -5966,6 +5976,26 @@ void testAddressChecksumValidation()
     EXPECT_EQ(bech32Hrp("abcdef1qpzry9x8gf2tvdw0s3jn54khce6mua7lmqqqxw"), std::string("abcdef"));
     EXPECT_EQ(bech32Hrp("A12UEL5L"), std::string("a"));    // lower-cased
     EXPECT_EQ(bech32Hrp("A12UEL5M"), std::string(""));     // invalid → empty
+
+    // Address type recognizers: accept every real DragonX recipient form, reject non-addresses.
+    using dragonx::util::isTransparentAddress;
+    using dragonx::util::isShieldedAddress;
+    using dragonx::util::isValidRecipientAddress;
+    const std::string p2pkh = "R9NXAVJezHiBnT3ijTpg3JUZre7PxhJWti";   // v60
+    const std::string p2sh  = "bCpbnCkrjoJ6EHXtLx9eASHEbFYyikt35C";   // v85 multisig — the regression
+    const std::string zaddr = "zs1qqqsyqcyq5rqwzqfpg9scrgwpugpzysnzs23v9ccrydpk8qarc0jqgfzyvjz2f389q5j5ctfvp5";
+    EXPECT_TRUE(isTransparentAddress(p2pkh));
+    EXPECT_TRUE(isTransparentAddress(p2sh));      // was silently dropped by the old 'R'-only filter
+    EXPECT_FALSE(isShieldedAddress(p2pkh));
+    EXPECT_TRUE(isShieldedAddress(zaddr));
+    EXPECT_FALSE(isTransparentAddress(zaddr));
+    EXPECT_TRUE(isValidRecipientAddress(p2pkh));
+    EXPECT_TRUE(isValidRecipientAddress(p2sh));
+    EXPECT_TRUE(isValidRecipientAddress(zaddr));
+    // A WIF spending key is NOT a recipient (33/34-byte payload, not 21); nor is a typo'd address.
+    EXPECT_FALSE(isTransparentAddress("Up3W7uVYkLxCfH91APxjSpkkGBWJyBrm3tt1bCz64V5fpZK9ef3C"));
+    EXPECT_FALSE(isValidRecipientAddress("R9NXAVJezHiBnT3ijTpg3JUZre7PxhJWtX"));  // flipped checksum char
+    EXPECT_FALSE(isValidRecipientAddress(""));
 }
 
 // Import-key recognition: the client gate must accept every real DragonX key form and reject