WebUI: per-signal vscale toolbar, active-signal highlighting, zoom fix

- Per-signal, per-plot vertical scale state (sigVScale keyed by plotId:signalKey)
  so the same signal in two plots has fully independent vscale config
- Active signal redrawn on top of all series with 2× line width for clear
  visual identification; badge click toggles selection and opens/closes the
  embedded vscale toolbar (click same badge again to deselect)
- Vscale configurator moved from floating popup to a slim toolbar strip
  anchored inside the plot card, with an × close button
- Trigger dropdown shows one entry per array signal with [0…N-1] label;
  opening it shows an index-picker dialog to choose the element
- Zoom resampling: when server returns no data for a zoomed range, fall
  back to the local circular buffer instead of returning empty arrays

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

Client/WebUI/main.go

@@ -43,8 +43,8 @@ func main() {
 		}
 	}
 	for _, arg := range sourceArgs {
-		label, addr := ParseSourceArg(arg)
-		sm.Add(label, addr, "", 0)
+		label, addr, mcastGroup, dataPort := ParseSourceArgFull(arg)
+		sm.Add(label, addr, mcastGroup, dataPort)
 	}
 
 	sub, err := fs.Sub(staticFiles, "static")