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/address_validation.h

@@ -35,5 +35,18 @@ bool isValidBech32(const std::string& s);
 // `s` is not valid Bech32. The HRP identifies the key/address type (e.g. "zivks").
 std::string bech32Hrp(const std::string& s);
 
+// True if `s` is a transparent (Base58Check) address — P2PKH *or* P2SH/multisig. Accepts any
+// address whose payload is a 21-byte version+hash160, so it covers both the 'R…' (v60) and 'b…'
+// (v85 script) forms on every DragonX network and rejects WIF keys / typos by checksum. Version-byte
+// agnostic by design — a bare prefix check ('R' only) silently drops valid P2SH recipients.
+bool isTransparentAddress(const std::string& s);
+
+// True if `s` is a shielded Sapling payment address (HRP "zs" / "ztestsapling" / "zregtestsapling"),
+// with a valid Bech32 checksum. Distinguishes a payment address from a viewing key (e.g. "zivks…").
+bool isShieldedAddress(const std::string& s);
+
+// True if `s` is any address a payment can be sent to (transparent or shielded).
+bool isValidRecipientAddress(const std::string& s);
+
 }  // namespace util
 }  // namespace dragonx