perf(chat): single-pass watermark and move-append on chat load

Loading chat history did more work than it needed to. Seeding the unread
watermark walked every conversation and copied each one's full message
list out of the store just to read the last timestamp, and the database
reload copied every message into the store one at a time with no
reservation.

- The watermark now comes from one pass over the stored messages that
  returns the latest timestamp per conversation, with no per-
  conversation copy or sort.
- The store gained a move-append and a reserve, so the reload pre-sizes
  its vector and dedup set once and moves each row in instead of copying
  it. The dedup key is computed before the move.

Behaviour is unchanged: the watermark values and the blocked-conversation
filter on reload are the same as before.

Tests cover the per-conversation latest-timestamp pass and move-append.

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

src/app_network.cpp

@@ -4091,10 +4091,8 @@ void App::provisionChatIdentityFromSecret(std::string secret)
             // Baseline unread: treat everything already in the store at load as read, so only messages
             // that arrive while the app is open badge as unread (Q1).
             const auto& store = chat_service_.store();
-            for (const auto& cid : store.conversationIds()) {
-                const auto msgs = store.conversation(cid);
-                if (!msgs.empty()) chat_seen_watermark_[cid] = msgs.back().timestamp;
-            }
+            for (const auto& [cid, ts] : store.latestTimestampByConversation())
+                chat_seen_watermark_[cid] = ts;
         }
     } else {
         chat_identity_unavailable_ = true;