feat(chat): per-conversation delete (revive + block); memoize chat/badge render

Adds a per-conversation "delete" with two modes, and removes the per-frame
rescans of the chat history that shared these files.

Delete conversation (header trash icon → confirm dialog):
- Delete (revive-on-new-message): clears local history and tombstones the
  messages (new chat_deleted table, keyed dedup hashes) so the every-few-
  seconds memo re-scan can't re-import them; a genuinely NEW message (new
  txid) revives the thread.
- Delete & block: removes history WITHOUT a tombstone and records the cid as
  blocked (settings); ChatService::ingest drops that conversation's messages
  — old and future — until unblocked from the "Blocked" manager, which then
  re-imports the conversation from chain.
- Local-only (messages remain on-chain; the peer keeps their copy).
  deleteConversation() deletes the DB rows FIRST and only then mutates the
  store, so a failed write can't leave the two diverged.
Unit-tested (revive / tombstone-survives-reload / block / unblock) and
adversarially reviewed (store/DB divergence, half-open DB, revive-unread).

Performance (chat + badge hot paths, from the perf audit):
- ChatStore gains revision(); the Chat unread badge is now a single O(N)
  no-alloc pass cached on it (was O(conversations x messages) copy+sort every
  frame). The conversation list and open thread are memoized on revision()
  (+ show-hidden and AddressBook::revision() for peer-name resolution).
- AddressBook gains revision() so an in-place contact rename invalidates the
  chat memo (an edit keeps entries().size() constant).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/chat/chat_store.cpp

@@ -13,6 +13,7 @@ std::string ChatStore::dedupKey(const ChatMessage& message) {
 bool ChatStore::append(const ChatMessage& message) {
     if (!seen_.insert(dedupKey(message)).second) return false;
     messages_.push_back(message);
+    ++revision_;
     return true;
 }
 
@@ -38,12 +39,24 @@ const ChatMessage* ChatStore::updateDelivery(const std::string& txid, ChatDelive
     for (auto& message : messages_) {
         if (message.txid == txid) {
             message.delivery = delivery;
+            ++revision_;
             return &message;
         }
     }
     return nullptr;
 }
 
+int ChatStore::countUnread(const std::function<bool(const std::string&)>& excluded,
+                           const std::function<std::int64_t(const std::string&)>& seenFor) const {
+    int unread = 0;
+    for (const auto& m : messages_) {
+        if (m.direction != ChatDirection::Incoming) continue;
+        if (excluded && excluded(m.conversation_id)) continue;
+        if (m.timestamp > seenFor(m.conversation_id)) ++unread;
+    }
+    return unread;
+}
+
 std::vector<std::string> ChatStore::conversationIds() const {
     std::vector<std::string> ids;
     std::unordered_set<std::string> seenIds;
@@ -53,9 +66,27 @@ std::vector<std::string> ChatStore::conversationIds() const {
     return ids;
 }
 
+std::vector<ChatMessage> ChatStore::eraseConversation(const std::string& conversationId) {
+    std::vector<ChatMessage> removed;
+    std::vector<ChatMessage> kept;
+    kept.reserve(messages_.size());
+    for (auto& m : messages_) {
+        if (m.conversation_id == conversationId) {
+            seen_.erase(dedupKey(m));
+            removed.push_back(std::move(m));
+        } else {
+            kept.push_back(std::move(m));
+        }
+    }
+    messages_.swap(kept);
+    if (!removed.empty()) ++revision_;
+    return removed;
+}
+
 void ChatStore::clear() {
     messages_.clear();
     seen_.clear();
+    ++revision_;
 }
 
 } // namespace dragonx::chat