feat(chat): show a progress hint while a tier-3 message is still arriving

Until now an incoming large message was invisible until every part landed.
ChatService::pendingLargeMessages() exposes the in-progress tier-3 receives
(conversation, chunks gathered / chunks needed) from the reassembly buffer, and
the chat tab renders a subtle centred line below the thread — "Receiving a large
message…  24 / 36 parts" — that updates each scan and disappears when the message
completes and surfaces as an ordinary bubble. A complete-but-withheld (sensitive
vault locked) group is not reported as incoming.

New string chat_receiving_large (English; 8 locales fall back). Unit test extended:
after the first slice the receive is reported in progress; after the whole message
arrives it is no longer pending.

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

src/util/i18n.cpp

@@ -297,6 +297,7 @@ void I18n::loadBuiltinEnglish()
         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_receiving_large"] = "Receiving a large message…";
         strings_["chat_mute"] = "Mute";
         strings_["chat_unmute"] = "Unmute";
         strings_["chat_hide"] = "Hide";