fix(workflows): disable start/end palette buttons when node already exists

Prevents users from adding duplicate start or end nodes, which made the
workflow invalid with no way to recover since those node kinds are
protected from deletion.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/renderer/components/WorkflowEditor.tsx

@@ -182,6 +182,13 @@ export function WorkflowEditor({
 
   const issues = validateWorkflowDefinition(activeWorkflow);
 
+  const disabledPaletteKinds = useMemo(() => {
+    const disabled = new Set<WorkflowNodeKind>();
+    if (activeWorkflow.graph.nodes.some((n) => n.kind === 'start')) disabled.add('start');
+    if (activeWorkflow.graph.nodes.some((n) => n.kind === 'end')) disabled.add('end');
+    return disabled;
+  }, [activeWorkflow.graph.nodes]);
+
   /* ── Change propagation ─────────────────────────────────── */
 
   function propagateActiveChange(updatedActive: WorkflowDefinition) {
@@ -482,7 +489,7 @@ export function WorkflowEditor({
       <div className="flex min-h-0 flex-1">
         {/* Left palette */}
         <div className="w-40 shrink-0 overflow-y-auto border-r border-[var(--color-border)] bg-[var(--color-surface-1)]">
-          <WorkflowNodePalette onAddNode={handleAddNode} />
+          <WorkflowNodePalette disabledKinds={disabledPaletteKinds} onAddNode={handleAddNode} />
         </div>
 
         {/* Center column: validation + canvas + settings */}