fix(rpc): refuse plaintext-remote RPC by default and tighten isLocalHost

F8 (security). Two related fixes to how the wallet decides whether an RPC target is
safe to send Basic-auth credentials to:

- isLocalHost() was matching any host that merely *starts* "127." via
  rfind("127.",0)==0, so "127.evil.com" (and "127.0.0.1.attacker", "127.300.0.1",
  "1270.0.0.1") were misclassified as loopback and treated as local. It now uses a
  strict isExactIPv4Loopback() parser: exactly four 0-255 dot-separated octets with
  the first == 127. localhost / ::1 / [::1] handling is unchanged.

- A remote rpchost over plain HTTP (no rpctls=1) previously only produced a
  dismissible warning and then sent rpcuser:rpcpassword in cleartext, where a
  local-network MITM could capture them. tryConnect() now REFUSES that connection
  (clear status line + one-time notification, no creds sent) unless the user opts in
  explicitly with rpcallowplaintext=1 in DRAGONX.conf (new
  ConnectionConfig::allow_plaintext_remote, parsed in parseConfFile; policy in the
  new allowsPlaintextRemote()). Local/embedded daemons and rpctls=1 remotes are
  unaffected.

BREAKING: a wallet configured for remote plaintext RPC will stop connecting until
rpcallowplaintext=1 (or rpctls=1) is added to DRAGONX.conf. Must be called out in the
release notes. The Settings-toggle UI is deferred (the conf-key opt-in is the recovery
path; see docs/daemon-startup-hardening.md).

Adds testIsLocalHost and testAllowsPlaintextRemote to test_phase4.cpp; one i18n key
(English) added to i18n.cpp.

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

src/util/i18n.cpp

@@ -1321,6 +1321,7 @@ void I18n::loadBuiltinEnglish()
         strings_["loading_stall_title"] = "Taking longer than expected";
         strings_["loading_stall_body"] = "The daemon has been initializing for %.0fs. This can be normal after an update or on first launch (loading the block index or rescanning) — it will connect automatically once ready.";
         strings_["loading_stall_hint"] = "Still stuck? Open Settings and use Restart Daemon, or check the Console for details.";
+        strings_["sb_plaintext_remote_blocked"] = "Refusing to send RPC credentials over plaintext to a remote host. Add rpcallowplaintext=1 to DRAGONX.conf to allow it, or enable TLS with rpctls=1.";
         strings_["sb_dragonxd_running"] = "dragonxd running";
         strings_["sb_dragonxd_stopping"] = "Stopping dragonxd...";
         strings_["sb_dragonxd_stopped"] = "dragonxd stopped";