cleanup: rebrand residual user-facing "HUSH" strings to DragonX

Sweeps the leftover coin-name strings in RPC help text, RPC output, and log
messages that the currency-unit change didn't cover:

- RPC help: "mining reward amount in HUSH" -> DRAGONX (mining.cpp x2);
  "at least minbal HUSH" -> DRAGONX (rpcwallet.cpp); "the HUSH address" /
  "(string) HUSH address" -> DragonX (rawtransaction.cpp)
- RPC output: the SMART_CHAIN_SYMBOL[0]==0 ? "HUSH" : SYMBOL coin-name fallback
  (crosschain/misc/mining/blockchain) -> "DRAGONX"; the notarizations JSON key
  make_pair("HUSH", ...) -> "DRAGONX"
- Logs: "HUSH blocktime changing", "stopping HUSH HTTP/REST/RPC",
  "HUSH raw magic=" -> DragonX

Left untouched (verified): the 82 "HUSH3"/ishush3 chain-symbol consensus checks;
hush_globals.h CURRENCIES[] price-oracle basket (internal lookup, dead feature
on DragonX); hush.h notarization debug printf; a commented-out cout in main.cpp.

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

src/wallet/rpcwallet.cpp

@@ -4049,7 +4049,7 @@ UniValue z_getbalances(const UniValue& params, bool fHelp, const CPubKey& mypk)
         throw runtime_error(
             "z_getbalances\n"
             "\nReturns array of addresses with their unspent shielded balances.\n"
-            "Optionally filter to only include addresses with at least minbal HUSH.\n"
+            "Optionally filter to only include addresses with at least minbal DRAGONX.\n"
             "Results are an array of Objects, each of which has:\n"
             "{address, balance}\n"
             "\nArguments:\n"