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/chat/chat_service.h

@@ -100,6 +100,16 @@ public:
     // local vault key; while that key is absent its messages are withheld from the store on ingest/load.
     bool isConversationSensitive(const std::string& conversationId) const;
 
+    // Larger-messages tier 3: a message still being received across transactions (some chunks arrived,
+    // not yet all). Lets the UI show an "incoming large message — N of M parts" hint until it completes
+    // and surfaces as an ordinary message. One entry per in-progress message; empty when none.
+    struct Tier3Progress {
+        std::string conversation_id;
+        int received = 0;   // chunks gathered so far
+        int total = 0;      // chunks the whole message needs
+    };
+    std::vector<Tier3Progress> pendingLargeMessages() const;
+
     // --- Outgoing (compose) ---
     // My chat public key (hex), or "" without an identity — goes in an outgoing header's "p".
     std::string identityPublicKeyHex() const;