mesh: bound the user-facing notification sprintf calls (#10437)

Two sites built ClientNotification messages with sprintf into a
fixed-size proto buffer with no length cap. The current format strings
fit comfortably, but a future caller editing either format string
without rechecking the buffer size would get a silent stack/heap
overrun. Switch to snprintf with sizeof so the bound is enforced at
the call site.

Co-authored-by: Ben Meadors <benmmeadors@gmail.com>

src/mesh/NodeDB.cpp

@@ -1881,7 +1881,7 @@ bool NodeDB::updateUser(uint32_t nodeId, meshtastic_User &p, uint8_t channelInde
                 meshtastic_ClientNotification *cn = clientNotificationPool.allocZeroed();
                 cn->level = meshtastic_LogRecord_Level_WARNING;
                 cn->time = getValidTime(RTCQualityFromNet);
-                sprintf(cn->message, warning, p.long_name);
+                snprintf(cn->message, sizeof(cn->message), warning, p.long_name);
                 service->sendClientNotification(cn);
             }
             return false;