ui: reword the docs that still spoke of the list drag; keep disabled signals' badge order; stamp the signal generation before the snapshot

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

help/C/org.udpscope.app/overview.page

@@ -92,7 +92,7 @@
         derived from it; <gui>Signals</gui> holds the trigger and the channel list, where a
         math channel is a row of that one list under a <gui>MATH</gui> heading whose
         <gui>+</gui> creates one. One row per channel: its colour, an enable switch, its name
-        and its unit. Channels are dragged from here onto a plot row.</p>
+        and its unit. A channel's badge in the badge bar is dragged onto a plot row.</p>
       </item>
       <item>
         <title>The plot</title>