Stop IRC by hand; remove mDNS

IrcService::disconnect() stops the session from any state: QUIT if
connected, then no more retries. /quit goes through it (before, it only
stopped a connected session; while waiting for Wi-Fi or retrying it did
nothing), and so does the new `irc stop` command. Stopped by hand,
opening the IRC App no longer reconnects; typing a line does.

mDNS is gone: it never crossed the dev box's routed network, and it
cost about 7.5 KB of RAM. Pushes go to the IP shown in Settings ->
Firmware, which drops its Name row. OTA Q54 records the change.

On the device, Debug Build: 105.6 KB free with Wi-Fi (was 98); with IRC
on TLS 54 KB free (was 46); after `irc stop`, back to 99 KB.

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

src/main.cpp

@@ -347,7 +347,7 @@ static const char* const kHelp =
     "coredump erase    forget the core dump in flash\n"
     "key <name|char>   press a key: up down left right select back home del tab, or one character\n"
     "wifi status | wifi add <ssid><TAB><password>\n"
-    "irc start | irc dump | irc say <buffer> <text>\n"
+    "irc start | irc stop | irc dump | irc say <buffer> <text>\n"
     "ls [folder] | rm <path> | install <path.ota> (Update from SD)\n"
     "sd list | cat <path> | log <text> | burst | sound on|off | short | normal\n"
 #ifdef RORO_DEBUG
@@ -463,6 +463,7 @@ static void runCommand(String line) {
         listingWanted = true;
     }
     if (line == "irc start") irc->connect();
+    if (line == "irc stop") irc->disconnect();
     if (line.startsWith("irc say ")) {  // irc say <buffer index> <text>
         String rest = line.substring(8);
         int space = rest.indexOf(' ');