feat(chat): raise the composer to the tier-3 cap + show the transaction count

Front end of tier-3 larger messages. The composer body cap goes from 4000 to
16000 bytes (kHushChatMaxMessageChunks = 64 chunks ⇒ up to 4 transactions), so a
message past the single-transaction ceiling now actually composes and sends via
the tier-3 path. The buffer and static_assert grow to match.

The beside-the-gauge indicator (and its hover tooltip) now distinguish the two
tiers: while a long message still fits one transaction it shows the part count as
before; once it spills across several it shows the TRANSACTION count — the real
cost the user asked to see — and the tooltip explains it is split across several
transactions, the recipient sees it only once every part has arrived, so it takes
longer to deliver and costs a fee per transaction. New strings chat_txns_label /
chat_parts_explain_multi (English; 8 locales fall back pending a translation pass).

Tier 3 is now end to end: compose → split → broadcast → reassemble → display.
A receive-side "incoming N/M parts" progress hint remains a future refinement
(an incomplete large message simply appears once whole).

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

src/util/i18n.cpp

@@ -294,7 +294,9 @@ void I18n::loadBuiltinEnglish()
         strings_["chat_export_confirm_btn"] = "Export";
         strings_["chat_len_over"] = "Message too long";
         strings_["chat_parts_label"] = "parts";
+        strings_["chat_txns_label"] = "txns";
         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_parts_explain_multi"] = "This message is too big for one transaction, so it is split across several. The recipient sees it once every part has arrived, so it takes longer to deliver and costs a network fee per transaction.";
         strings_["chat_mute"] = "Mute";
         strings_["chat_unmute"] = "Unmute";
         strings_["chat_hide"] = "Hide";