fix(send): validate recipient address checksums (Base58Check + Bech32)

The send screen labelled any prefix+length match as a "Valid" address, so a
mistyped address that still matched the pattern passed the gate. Add pure,
offline checksum validation — Base58Check (transparent R-addresses) and Bech32
(Sapling zs-addresses) — and require it in the validity check. Both verifiers are
version-byte/HRP agnostic (the HRP is taken from the string, the Base58 checksum
is chain-independent), so a correct implementation never rejects a genuine
address while catching transcription errors. Works for both build variants
(no daemon round-trip), unit-tested against standard BIP173 / Base58Check vectors.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/util/address_validation.h

@@ -0,0 +1,28 @@
+// DragonX Wallet - ImGui Edition
+// Copyright 2024-2026 The Hush Developers
+// Released under the GPLv3
+//
+// address_validation.h — pure, offline checksum validation for wallet addresses.
+// These verify the *encoding checksum* (Base58Check / Bech32), which is independent
+// of the chain's version bytes / HRP, so a correct implementation never rejects a
+// genuinely-valid address while still catching transcription errors. Used to make
+// the send screen's "Valid address" label/gate reflect reality instead of a bare
+// prefix+length heuristic. No network, no daemon — safe for both build variants.
+
+#pragma once
+
+#include <string>
+
+namespace dragonx {
+namespace util {
+
+// True if `s` decodes as Base58Check with a valid 4-byte double-SHA256 checksum
+// (transparent R-addresses). Version-byte agnostic by design.
+bool isValidBase58Check(const std::string& s);
+
+// True if `s` is a valid Bech32 string (Sapling zs-addresses). The HRP is taken
+// from the string itself and folded into the checksum, so no HRP is hardcoded.
+bool isValidBech32(const std::string& s);
+
+}  // namespace util
+}  // namespace dragonx