fix(ui): re-measure autocomplete scrollbar inset on programmatic value changes

The --lm-vscrollbar-width inset from 634ea7f2 was only refreshed on input
events, mount and mode changes. Programmatic value updates (widget.setValue
from "send lora to workflow", external value-change events) change the
textarea content without an input event, leaving the corner clear (x)
button overlapping a freshly appeared classic scrollbar until the next
keystroke. Mount-time pending value replay was already covered.

- onExternalValueChange and widget.onSetValue now call
  updateVScrollbarWidth() alongside the hasText update
- tests: cover both paths by overriding textarea metrics to an overflowing
  state and asserting the 15px gutter lands in the CSS var

vue-widgets/src/components/AutocompleteTextWidget.vue

@@ -200,6 +200,9 @@ const onWheel = (event: WheelEvent) => {
 
 // Handle external value changes (e.g., from "send lora to workflow")
 const onExternalValueChange = () => {
+  // The DOM value was set synchronously by the caller; the new content may
+  // have toggled the vertical scrollbar on/off.
+  updateVScrollbarWidth()
   updateHasTextState()
 }
 
@@ -209,6 +212,9 @@ const setupWidgetOnSetValue = () => {
     props.widget.onSetValue = (value: string) => {
       // The DOM value is already set by setValue, just update our state
       hasText.value = value.length > 0
+      // Programmatic sets can toggle the scrollbar; re-measure so the
+      // corner clear button stays clear of it.
+      updateVScrollbarWidth()
     }
   }
 }