feat(chat): show the on-chain part count for a long message in the composer

A larger message (tier 2) is carried as one transaction split across up to 16
memo outputs, but the composer gave no sign of that. It now shows the part
count beside the byte gauge the moment the body spans more than one memo output
(computed the same way buildOutgoingMessage chunks it), and the gauge's hover
tooltip adds the count plus "sent as one transaction split into these memo
parts — it arrives as quickly as a normal message" (the honest answer: it's one
atomic 0-conf tx, not N transactions — multi-transaction splitting is the
unbuilt tier 3). New strings chat_parts_label / chat_parts_explain (English;
8 locales fall back).

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

src/util/i18n.cpp

@@ -293,6 +293,8 @@ void I18n::loadBuiltinEnglish()
         strings_["chat_export_confirm_path"] = "Will be saved to:";
         strings_["chat_export_confirm_btn"] = "Export";
         strings_["chat_len_over"] = "Message too long";
+        strings_["chat_parts_label"] = "parts";
+        strings_["chat_parts_explain"] = "This long message is sent as one transaction split into these memo parts — it arrives as quickly as a normal message.";
         strings_["chat_mute"] = "Mute";
         strings_["chat_unmute"] = "Unmute";
         strings_["chat_hide"] = "Hide";