perf(chat): look up chat txids through a revision-keyed index

The History tab copies every chat txid into a fresh set each frame to
badge and filter chat transactions: M string allocations for M chat
messages. ChatStore gains hasChatTxid(), an O(1) lookup in a txid index
that is rebuilt only after the store changed (append, delivery update,
erase, clear) and never built for an empty store. clear() also drops
the index, so no chat txids linger after a lock. The History tab moves
to it in the next commit.

The chat note-buffer pump, which runs every frame while chat is idle,
built the whole conversation-id list just to see whether the store was
empty; it now asks store().empty(), O(1) instead of O(M).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/chat/chat_store.cpp

@@ -90,6 +90,7 @@ std::vector<ChatMessage> ChatStore::eraseConversation(const std::string& convers
 void ChatStore::clear() {
     messages_.clear();
     seen_.clear();
+    txid_index_.clear();   // drop the History-badge txid copies too (revision_ below invalidates the memo)
     ++revision_;
 }