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.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