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/config/settings.cpp

@@ -157,6 +157,13 @@ bool Settings::load(const std::string& path)
             for (const auto& c : j["hidden_chat_cids"])
                 if (c.is_string()) hidden_chat_cids_.push_back(c.get<std::string>());
         }
+        if (j.contains("blocked_chat_convs") && j["blocked_chat_convs"].is_array()) {
+            blocked_chat_convs_.clear();
+            for (const auto& c : j["blocked_chat_convs"])
+                if (c.is_object() && c.contains("cid") && c["cid"].is_string())
+                    blocked_chat_convs_.push_back({c["cid"].get<std::string>(),
+                                                   c.value("name", std::string())});
+        }
         // Chat-tab customization (re-clamped through the setters so hand-edited JSON stays in range).
         loadScalar(j, "chat_emoji_color", chat_emoji_color_);
         loadScalar(j, "chat_poll_rate_sec", chat_poll_rate_sec_); setChatPollRateSec(chat_poll_rate_sec_);
@@ -461,6 +468,13 @@ bool Settings::save(const std::string& path)
     j["hidden_chat_cids"] = json::array();
     for (const auto& c : hidden_chat_cids_)
         j["hidden_chat_cids"].push_back(c);
+    j["blocked_chat_convs"] = json::array();
+    for (const auto& b : blocked_chat_convs_) {
+        json o;
+        o["cid"] = b.cid;
+        o["name"] = b.name;
+        j["blocked_chat_convs"].push_back(o);
+    }
     j["chat_emoji_color"] = chat_emoji_color_;
     j["chat_poll_rate_sec"] = chat_poll_rate_sec_;
     j["chat_bubble_style"] = chat_bubble_style_;