Slack: transcript rows get their own class; Names switch works with the new rows; notices switch renamed

The row shared its class with the Send tile's status line, so every
transcript row was drawn at 0.8em, muted and nowrap; rows are .sl-row now
and read at the tile's text size. The Names switch targeted the old
markup (name line directly in the row) and hid nothing once the rows
gained a body. "System lines" reads "Channel notices", with a help line
naming what those are.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

modules/slack/module.json

@@ -117,8 +117,9 @@
     },
     "showSystem": {
       "type": "switch",
-      "label": "System lines",
-      "default": true
+      "label": "Channel notices",
+      "default": true,
+      "help": "Joins, leaves, topic changes, pins and other housekeeping lines."
     }
   }
 }