feat(chat): mute conversations (Q10)

Per-conversation mute toggle in the thread header. Muted conversations (tracked
by cid in settings, so it persists) are skipped by chatUnreadCount(), so they
neither raise the nav-item unread badge nor the new-message toast — the toast now
gates on a chatUnreadCount() delta across ingest, which already skips muted cids,
so mute is respected for free. "Block" (rejecting a peer's inbound memos) is a
larger ingest-filter change and is intentionally left out of this pass.

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

src/config/settings.h

@@ -118,6 +118,18 @@ public:
     std::string getChatReplyZaddr() const { return chat_reply_zaddr_; }
     void setChatReplyZaddr(const std::string& z) { chat_reply_zaddr_ = z; }
 
+    // Muted chat conversations (by cid) — muted conversations don't badge or raise a toast (Q10).
+    bool isChatMuted(const std::string& cid) const {
+        return std::find(muted_chat_cids_.begin(), muted_chat_cids_.end(), cid) != muted_chat_cids_.end();
+    }
+    void setChatMuted(const std::string& cid, bool muted) {
+        const bool already = isChatMuted(cid);
+        if (muted && !already) muted_chat_cids_.push_back(cid);
+        else if (!muted && already)
+            muted_chat_cids_.erase(std::remove(muted_chat_cids_.begin(), muted_chat_cids_.end(), cid),
+                                   muted_chat_cids_.end());
+    }
+
     // Privacy
     bool getSaveZtxs() const { return save_ztxs_; }
     void setSaveZtxs(bool save) { save_ztxs_ = save; }
@@ -473,6 +485,7 @@ private:
     std::string theme_ = "dragonx";
     std::string skin_id_ = "dragonx";
     std::string chat_reply_zaddr_;
+    std::vector<std::string> muted_chat_cids_;   // muted chat conversations by cid (Q10)
     bool save_ztxs_ = true;
     bool auto_shield_ = true;
     bool use_tor_ = false;