feat: Add custom step continuation flow with save/use/branch options

Custom steps during tree navigation now support a complete workflow:
- PostStepActionModal: Save for Later / Use Now / Both options
- ContinuationModal: Pick descendant nodes or build custom branch
- ForkTreeModal: Save modified tree as personal copy at completion
- Custom steps are recorded in decisions array for export
- Fix popular-tags API endpoint URL mismatch
- Add aria-labels for accessibility on select/button elements

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

frontend/src/components/session/ContinuationModal.tsx

@@ -0,0 +1,139 @@
+import { Modal } from '@/components/common/Modal'
+import { ArrowRight, GitBranch, AlertTriangle, HelpCircle, Zap, CheckCircle } from 'lucide-react'
+import { cn } from '@/lib/utils'
+import type { NodeType } from '@/types/tree'
+
+export interface DescendantNode {
+  id: string
+  label: string
+  type: NodeType
+  parentOptionLabel: string
+}
+
+interface ContinuationModalProps {
+  isOpen: boolean
+  onClose: () => void
+  descendantNodes: DescendantNode[]
+  onSelectNode: (nodeId: string) => void
+  onBuildCustomBranch: () => void
+}
+
+const nodeTypeIcons: Record<NodeType, React.ReactNode> = {
+  decision: <HelpCircle className="h-4 w-4 text-blue-500" />,
+  action: <Zap className="h-4 w-4 text-amber-500" />,
+  solution: <CheckCircle className="h-4 w-4 text-green-500" />
+}
+
+const nodeTypeLabels: Record<NodeType, string> = {
+  decision: 'Decision',
+  action: 'Action',
+  solution: 'Solution'
+}
+
+export function ContinuationModal({
+  isOpen,
+  onClose,
+  descendantNodes,
+  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 (
+    <Modal isOpen={isOpen} onClose={onClose} title="Where does this lead?" size="lg">
+      <div className="space-y-6">
+        {/* Descendant Selection */}
+        {hasDescendants && (
+          <div>
+            <p className="mb-4 text-sm text-muted-foreground">
+              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>
+                </div>
+              ))}
+            </div>
+          </div>
+        )}
+
+        {/* Divider */}
+        {hasDescendants && (
+          <div className="flex items-center gap-4">
+            <div className="h-px flex-1 bg-border" />
+            <span className="text-xs font-medium uppercase tracking-wide text-muted-foreground">
+              Or
+            </span>
+            <div className="h-px flex-1 bg-border" />
+          </div>
+        )}
+
+        {/* Build Custom Branch */}
+        <div>
+          <button
+            onClick={onBuildCustomBranch}
+            className={cn(
+              'flex w-full items-center gap-3 rounded-lg border-2 border-dashed border-amber-500/50 bg-amber-500/5 p-4 text-left transition-colors',
+              'hover:border-amber-500 hover:bg-amber-500/10'
+            )}
+          >
+            <div className="flex h-10 w-10 flex-shrink-0 items-center justify-center rounded-full bg-amber-500/10">
+              <GitBranch className="h-5 w-5 text-amber-500" />
+            </div>
+            <div className="flex-1">
+              <p className="font-medium">Build Custom Branch</p>
+              <p className="text-sm text-muted-foreground">
+                Create your own troubleshooting path with custom steps
+              </p>
+            </div>
+          </button>
+
+          {/* Warning */}
+          <div className="mt-3 flex items-start gap-2 rounded-md bg-amber-500/10 p-3">
+            <AlertTriangle className="mt-0.5 h-4 w-4 flex-shrink-0 text-amber-500" />
+            <p className="text-sm text-amber-700 dark:text-amber-400">
+              You'll need to complete this branch manually or mark the issue as resolved.
+              Custom branches can be saved as a personal tree when your session ends.
+            </p>
+          </div>
+        </div>
+      </div>
+    </Modal>
+  )
+}