fix(ui): validate shielded addresses with the correct RPC

The Validate address dialog chose its RPC from the first character of
the input, then asked the transparent-only validateaddress for every
case. That RPC is Base58-only, so a valid Sapling (z) address came back
isvalid=false and was reported invalid.

- The input is now classified with the checksum-verified recognizers,
  and a shielded address is sent to z_validateaddress while a
  transparent (mining) address keeps validateaddress.
- Anything that is not an address - a typo, garbage, or a pasted
  viewing/spending key - is judged invalid on the spot and never sent to
  the node, so a key pasted into the field is not leaked to an RPC.
- On Lite there is no node to ask, so the dialog shows the checksum
  verdict alone and hides the "owned by this wallet" row.

Tests: validateRpcMethod maps shielded to z_validateaddress, transparent
to validateaddress, and a key or empty string to no RPC.

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

src/util/address_validation.cpp

@@ -198,5 +198,15 @@ RecipientKind classifyRecipient(const std::string& s)
     return RecipientKind::Invalid;
 }
 
+const char* validateRpcMethod(RecipientKind kind)
+{
+    switch (kind) {
+    case RecipientKind::Shielded:   return "z_validateaddress";
+    case RecipientKind::MiningOnly: return "validateaddress";
+    case RecipientKind::Invalid:    break;
+    }
+    return nullptr;
+}
+
 }  // namespace util
 }  // namespace dragonx