fix(import): recognize real DragonX key formats in the import gate

The client-side pre-check rejected legitimate keys before the daemon ever
saw them, surfacing "Unrecognized key format" / a cryptic daemon "Invalid"
error. Two concrete defects plus the brittle heuristic behind them:

- Viewing keys: isViewingKey looked for Zcash's "zxview" extended-FVK
  prefix, but DragonX's z_exportviewingkey emits a Sapling *incoming*
  viewing key (HRP "zivks"), which z_importviewingkey is the only form the
  daemon decodes. Every real DragonX viewing key was refused. (F1)
- Uncompressed transparent WIF: the length+first-char heuristic accepted
  {5,K,L,U} only, but a version-188 uncompressed key starts with '7'. (F2)

Replace the heuristic with structural validation using the existing
checksum validators (F3): add util::decodeBase58Check (checksum-stripped
payload) and util::bech32Hrp (HRP of a valid Bech32 string). Transparent
keys are now accepted by decoding Base58Check and checking the payload is a
33/34-byte secret key with a DragonX SECRET_KEY version byte (188 main/
regtest, 128 testnet) — covering compressed and uncompressed, rejecting
addresses/typos by real checksum. Viewing keys are matched by the real
incoming-VK HRPs (zivks / zivktestsapling / zivkregtestsapling).

The Sweep gate and the dialog's live type indicator run off the same
predicates, so they are fixed too (F4). Messaging now names the likely
cause and appends a wrong-coin/network hint to the daemon's raw "Invalid"
error (F5).

Adds testPrivateKeyImportRecognition plus decodeBase58Check/bech32Hrp
coverage; suite green (1/1).

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

src/util/address_validation.cpp

@@ -76,7 +76,7 @@ std::vector<int> bech32HrpExpand(const std::string& hrp)
 
 }  // namespace
 
-bool isValidBase58Check(const std::string& s)
+bool decodeBase58Check(const std::string& s, std::vector<std::uint8_t>& payloadOut)
 {
     if (s.size() < 5 || s.size() > 256) return false;
     std::vector<std::uint8_t> data;
@@ -88,7 +88,15 @@ bool isValidBase58Check(const std::string& s)
     unsigned char h2[crypto_hash_sha256_BYTES];
     crypto_hash_sha256(h1, data.data(), payloadLen);
     crypto_hash_sha256(h2, h1, sizeof(h1));
-    return std::memcmp(h2, data.data() + payloadLen, 4) == 0;
+    if (std::memcmp(h2, data.data() + payloadLen, 4) != 0) return false;
+    payloadOut.assign(data.begin(), data.begin() + payloadLen);
+    return true;
+}
+
+bool isValidBase58Check(const std::string& s)
+{
+    std::vector<std::uint8_t> payload;
+    return decodeBase58Check(s, payload);
 }
 
 bool isValidBech32(const std::string& s)
@@ -127,5 +135,18 @@ bool isValidBech32(const std::string& s)
     return bech32Polymod(combined) == 1;  // original Bech32 constant (Sapling, not Bech32m)
 }
 
+std::string bech32Hrp(const std::string& s)
+{
+    if (!isValidBech32(s)) return {};
+    // isValidBech32 already rejected mixed case and guaranteed a non-empty HRP before the
+    // final '1' separator, so lower-casing and splitting there recovers the HRP verbatim.
+    std::string lower(s);
+    std::transform(lower.begin(), lower.end(), lower.begin(),
+                   [](unsigned char c) { return static_cast<char>(std::tolower(c)); });
+    const std::size_t sep = lower.rfind('1');
+    if (sep == std::string::npos) return {};
+    return lower.substr(0, sep);
+}
+
 }  // namespace util
 }  // namespace dragonx

src/util/address_validation.h

@@ -11,7 +11,9 @@
 
 #pragma once
 
+#include <cstdint>
 #include <string>
+#include <vector>
 
 namespace dragonx {
 namespace util {
@@ -20,9 +22,18 @@ namespace util {
 // (transparent R-addresses). Version-byte agnostic by design.
 bool isValidBase58Check(const std::string& s);
 
+// Decodes `s` as Base58Check; on success returns true and fills `payloadOut` with the
+// decoded bytes EXCLUDING the trailing 4-byte checksum (i.e. version byte + data). Lets
+// callers inspect the version byte / payload length (e.g. to tell a WIF from an address).
+bool decodeBase58Check(const std::string& s, std::vector<std::uint8_t>& payloadOut);
+
 // 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);
 
+// Returns the (lower-cased) human-readable prefix of a valid Bech32 string, or "" if
+// `s` is not valid Bech32. The HRP identifies the key/address type (e.g. "zivks").
+std::string bech32Hrp(const std::string& s);
+
 }  // namespace util
 }  // namespace dragonx