feat: add public SharedSessionPage with tree preview

Add the public-facing shared session page at /share/:shareToken that
renders shared sessions without authentication. Includes error handling
for 401 (redirect to login), 403 (access denied), 404 (not found),
and 410 (expired). The page features a minimal header, session metadata,
SessionTimeline component, and a new SharedSessionTreePreview component
that renders the decision tree structure with the path taken highlighted.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/session/SharedSessionTreePreview.tsx

@@ -0,0 +1,88 @@
+import { cn } from '@/lib/utils'
+
+interface SharedSessionTreePreviewProps {
+  treeStructure: Record<string, unknown>
+  pathTaken: string[]
+}
+
+const nodeTypeColors: Record<string, string> = {
+  root: 'bg-white',
+  decision: 'bg-blue-400',
+  action: 'bg-yellow-400',
+  solution: 'bg-emerald-400',
+  information: 'bg-white/50',
+}
+
+function getNodeTitle(node: Record<string, unknown>): string {
+  return (
+    (node.question as string) ||
+    (node.title as string) ||
+    (node.node_type as string) ||
+    'Untitled'
+  )
+}
+
+function TreeNode({
+  node,
+  depth,
+  pathTaken,
+}: {
+  node: Record<string, unknown>
+  depth: number
+  pathTaken: string[]
+}) {
+  const nodeId = (node.id as string) || ''
+  const nodeType = (node.node_type as string) || 'decision'
+  const isInPath = pathTaken.includes(nodeId)
+  const children = (node.children as Record<string, unknown>[]) || []
+  const colorClass = nodeTypeColors[nodeType] || 'bg-white/50'
+
+  return (
+    <>
+      <div
+        className={cn(
+          'flex items-center gap-2 px-3 py-1.5 text-sm',
+          isInPath
+            ? 'rounded-md border-l-2 border-white/40 bg-white/10 font-medium text-white'
+            : 'text-white/30'
+        )}
+        style={{ paddingLeft: `${depth * 16 + 12}px` }}
+      >
+        <span
+          className={cn('h-2 w-2 shrink-0 rounded-full', colorClass)}
+        />
+        <span className="truncate">{getNodeTitle(node)}</span>
+      </div>
+      {children.map((child, index) => (
+        <TreeNode
+          key={(child.id as string) || index}
+          node={child}
+          depth={depth + 1}
+          pathTaken={pathTaken}
+        />
+      ))}
+    </>
+  )
+}
+
+export function SharedSessionTreePreview({
+  treeStructure,
+  pathTaken,
+}: SharedSessionTreePreviewProps) {
+  if (!treeStructure) {
+    return null
+  }
+
+  return (
+    <div className="glass-card rounded-2xl">
+      <div className="sticky top-0 z-10 rounded-t-2xl border-b border-white/[0.06] bg-black/80 px-6 py-4 backdrop-blur">
+        <h3 className="text-sm font-semibold text-white">Tree Structure</h3>
+      </div>
+      <div className="max-h-[600px] overflow-y-auto py-2">
+        <TreeNode node={treeStructure} depth={0} pathTaken={pathTaken} />
+      </div>
+    </div>
+  )
+}
+
+export default SharedSessionTreePreview