fix: ContinuationModal UX - hover tooltips and stay-on-step flow

- Replace grouped section headers with hover tooltips (title attr) for
  a cleaner flat list of descendant options
- After selecting a descendant, stay on the custom step so the user can
  write notes before proceeding via a "Continue to" button
- Add pendingContinuationNodeId state to track selected descendant
- "Continue to" and custom branch controls are mutually exclusive

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/components/session/ContinuationModal.tsx

@@ -37,15 +37,6 @@ export function ContinuationModal({
   onSelectNode,
   onBuildCustomBranch
 }: ContinuationModalProps) {
-  // Group nodes by parent option
-  const groupedNodes = descendantNodes.reduce((acc, node) => {
-    if (!acc[node.parentOptionLabel]) {
-      acc[node.parentOptionLabel] = []
-    }
-    acc[node.parentOptionLabel].push(node)
-    return acc
-  }, {} as Record<string, DescendantNode[]>)
-
   const hasDescendants = descendantNodes.length > 0
 
   return (
@@ -58,36 +49,28 @@ export function ContinuationModal({
               Select the next step in your troubleshooting path:
             </p>
 
-            <div className="space-y-4">
-              {Object.entries(groupedNodes).map(([parentOption, nodes]) => (
-                <div key={parentOption}>
-                  <p className="mb-2 text-xs font-medium uppercase tracking-wide text-muted-foreground">
-                    From: {parentOption}
-                  </p>
-                  <div className="space-y-2">
-                    {nodes.map(node => (
-                      <button
-                        key={node.id}
-                        onClick={() => onSelectNode(node.id)}
-                        className={cn(
-                          'flex w-full items-center gap-3 rounded-lg border border-border p-3 text-left transition-colors',
-                          'hover:border-primary hover:bg-accent'
-                        )}
-                      >
-                        <div className="flex h-8 w-8 flex-shrink-0 items-center justify-center rounded-full bg-muted">
-                          {nodeTypeIcons[node.type]}
-                        </div>
-                        <div className="min-w-0 flex-1">
-                          <p className="truncate font-medium">{node.label}</p>
-                          <p className="text-xs text-muted-foreground">
-                            {nodeTypeLabels[node.type]}
-                          </p>
-                        </div>
-                        <ArrowRight className="h-4 w-4 flex-shrink-0 text-muted-foreground" />
-                      </button>
-                    ))}
+            <div className="space-y-2">
+              {descendantNodes.map(node => (
+                <button
+                  key={node.id}
+                  onClick={() => onSelectNode(node.id)}
+                  title={`From: ${node.parentOptionLabel}`}
+                  className={cn(
+                    'flex w-full items-center gap-3 rounded-lg border border-border p-3 text-left transition-colors',
+                    'hover:border-primary hover:bg-accent'
+                  )}
+                >
+                  <div className="flex h-8 w-8 flex-shrink-0 items-center justify-center rounded-full bg-muted">
+                    {nodeTypeIcons[node.type]}
                   </div>
-                </div>
+                  <div className="min-w-0 flex-1">
+                    <p className="truncate font-medium">{node.label}</p>
+                    <p className="text-xs text-muted-foreground">
+                      {nodeTypeLabels[node.type]}
+                    </p>
+                  </div>
+                  <ArrowRight className="h-4 w-4 flex-shrink-0 text-muted-foreground" />
+                </button>
               ))}
             </div>
           </div>