IRC: edit server settings as a draft, applied under the service lock

The IRC App edited the live config while the IRC task could be reading
it to connect. The App now edits a copy; applyConfig() validates it and
swaps it in under the lock, and the task connects from a snapshot.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EhqxQ49eCju4CzKYNjZzwT

src/services/irc_service.h

@@ -44,9 +44,10 @@ class IrcService : public Service {
     }
     int totalUnread();
 
-    IrcConfig& config() { return config_; }
-    // Saves the config and, if running, reconnects with it (a fresh session).
-    std::string saveConfig();
+    // A copy of the server settings to edit; applyConfig() validates, saves and, if running,
+    // reconnects with it (a fresh session).
+    IrcConfig draftConfig();
+    std::string applyConfig(const IrcConfig& draft);
 
    private:
     struct Lock {