Make a call node say why it does nothing, and easier to place

A behavior node that cannot run looked exactly like a healthy one: no
start node, no behavior picked, a deleted behavior or a self-call were
all silently skipped. The call still had to be forgiving at runtime -- a
dangling reference must not abort a game -- so the fix is diagnosis, not
an exception: describeProblem() names the one reason, the node draws it
in red under its title, and the inspector repeats it where you pick the
behavior. A missing end node is deliberately not a problem: a call is
always synchronous, and the end node only decides which values come back.

Two ways in: a behavior dragged from the asset panel lands in the graph
under the pointer, and double-clicking a call node opens the graph it
runs. Both reuse machinery that already existed -- the asset panel
already starts the drag, and list views already report double clicks.

The e2e module needed its assets at the head of the asset list: after 18
modules the list is long enough that a newly appended row scrolls out of
view, and the press then lands on the panel behind it, starting no drag
at all. That looked exactly like a rejected drop and cost two full runs.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/ui/Theme.ts

@@ -73,6 +73,7 @@ export type TextStyleToken =
   | 'muted'
   | 'override'
   | 'small'
+  | 'warning'
   | 'title'
   | 'mono'
   | 'accent'
@@ -100,6 +101,9 @@ export function textStyle(token: TextStyleToken = 'body'): TextStyle {
     case 'small':
       style = new TextStyle({ ...base, fontSize: Theme.smallFontSize, fill: Theme.colors.textMuted });
       break;
+    case 'warning':
+      style = new TextStyle({ ...base, fontSize: Theme.smallFontSize, fill: Theme.colors.danger });
+      break;
     case 'title':
       style = new TextStyle({
         ...base,