feat(collections): show real document hierarchy in sidebar outline

Builds a tree from parentDocumentId over the flat documents.list
response instead of a flat list — collections.documents returns this
shape server-side already, but its NavigationNode has no emoji or
timestamps, which would break per-row icons and date-based sidebar
sort. Each node gets its own expand/collapse now.

Outpost/Features/Collections/CollectionDocumentsOutline.swift

@@ -2,17 +2,17 @@
 import SwiftUI
 import OutlineKit
 
-/// Flat document list nested under an expanded collection in the sidebar tree.
-/// Documents can themselves have children (`parentDocumentId`) in Outline — not
-/// represented here yet, this is a first pass at the collection-level expansion.
+/// Nested document outline under an expanded collection in the sidebar tree —
+/// real hierarchy via `parentDocumentId`, see DocumentNode for why that's
+/// client-side rather than `collections.documents`.
 struct CollectionDocumentsOutline: View {
     @State private var viewModel: DocumentsViewModel
     let sortOption: SidebarSortOption
     let refreshToken: Int
     let onSelectDocument: (OutlineDocument) -> Void
 
-    private var sortedDocuments: [OutlineDocument] {
-        sortOption.sorted(viewModel.documents)
+    private var tree: [DocumentNode] {
+        buildDocumentTree(from: viewModel.documents, sortedBy: sortOption)
     }
 
     init(
@@ -40,33 +40,71 @@ struct CollectionDocumentsOutline: View {
                     .foregroundStyle(.secondary)
                     .padding(.vertical, 6)
             } else {
-                ForEach(sortedDocuments) { document in
-                    Button {
-                        onSelectDocument(document)
-                    } label: {
-                        HStack(spacing: 8) {
-                            if let emoji = document.emoji {
-                                Text(emoji)
-                                    .font(.body)
-                            } else {
-                                Image(systemName: "doc.text")
-                                    .font(.callout)
-                                    .foregroundStyle(.secondary)
-                            }
-                            Text(document.title.isEmpty ? "Untitled" : document.title)
-                                .font(.body)
-                                .lineLimit(1)
-
-                            Spacer(minLength: 0)
-                        }
-                        .contentShape(Rectangle())
-                    }
-                    .buttonStyle(.plain)
-                    .padding(.vertical, 6)
+                ForEach(tree) { node in
+                    DocumentNodeRow(node: node, depth: 0, onSelectDocument: onSelectDocument)
                 }
             }
         }
         .task(id: refreshToken) { await viewModel.load() }
     }
 }
+
+private struct DocumentNodeRow: View {
+    let node: DocumentNode
+    let depth: Int
+    let onSelectDocument: (OutlineDocument) -> Void
+
+    @State private var isExpanded = false
+
+    var body: some View {
+        VStack(alignment: .leading, spacing: 0) {
+            HStack(spacing: 6) {
+                if node.children.isEmpty {
+                    Color.clear.frame(width: 12)
+                } else {
+                    Button {
+                        isExpanded.toggle()
+                    } label: {
+                        Image(systemName: "chevron.right")
+                            .font(.caption2.weight(.semibold))
+                            .foregroundStyle(.secondary)
+                            .rotationEffect(.degrees(isExpanded ? 90 : 0))
+                            .frame(width: 12)
+                    }
+                    .buttonStyle(.plain)
+                }
+
+                Button {
+                    onSelectDocument(node.document)
+                } label: {
+                    HStack(spacing: 8) {
+                        if let emoji = node.document.emoji {
+                            Text(emoji)
+                                .font(.body)
+                        } else {
+                            Image(systemName: "doc.text")
+                                .font(.callout)
+                                .foregroundStyle(.secondary)
+                        }
+                        Text(node.document.title.isEmpty ? "Untitled" : node.document.title)
+                            .font(.body)
+                            .lineLimit(1)
+
+                        Spacer(minLength: 0)
+                    }
+                    .contentShape(Rectangle())
+                }
+                .buttonStyle(.plain)
+            }
+            .padding(.vertical, 6)
+            .padding(.leading, CGFloat(depth) * 16)
+
+            if isExpanded {
+                ForEach(node.children) { child in
+                    DocumentNodeRow(node: child, depth: depth + 1, onSelectDocument: onSelectDocument)
+                }
+            }
+        }
+    }
+}
 #endif

Outpost/Features/Collections/DocumentNode.swift

@@ -0,0 +1,26 @@
+import Foundation
+import OutlineKit
+
+/// Reconstructs the real document hierarchy client-side from `parentDocumentId`.
+/// `documents.list` already returns the full flat set for a collection
+/// (including children, not just roots), so no separate tree-shaped endpoint
+/// is needed — `collections.documents` exists and returns exactly this shape,
+/// but its `NavigationNode` carries no emoji/timestamps, which would break
+/// per-row icons and date-based sidebar sorting.
+struct DocumentNode: Identifiable {
+    let document: OutlineDocument
+    let children: [DocumentNode]
+    var id: String { document.id }
+}
+
+func buildDocumentTree(from documents: [OutlineDocument], sortedBy sort: SidebarSortOption) -> [DocumentNode] {
+    let byParent = Dictionary(grouping: documents, by: { $0.parentDocumentId })
+
+    func makeNodes(parentId: String?) -> [DocumentNode] {
+        sort.sorted(byParent[parentId] ?? []).map { document in
+            DocumentNode(document: document, children: makeNodes(parentId: document.id))
+        }
+    }
+
+    return makeNodes(parentId: nil)
+}