feat: markdown preview mode in the Mac editor

ios/KeepMac/MacMarkdownView.swift

@@ -0,0 +1,128 @@
+import SwiftUI
+
+/// Read-only markdown rendering for the Mac editor's preview mode, standing in
+/// for the web's react-markdown pane. Parses with Foundation's markdown
+/// support and lays out block intents (headings, lists, quotes, code fences)
+/// that `Text` alone won't style.
+struct MacMarkdownView: View {
+    let source: String
+
+    var body: some View {
+        ScrollView {
+            VStack(alignment: .leading, spacing: 12) {
+                ForEach(blocks) { block in
+                    view(for: block)
+                }
+            }
+            .frame(maxWidth: .infinity, alignment: .leading)
+            .padding(.vertical, 8)
+        }
+    }
+
+    private struct Block: Identifiable {
+        enum Kind {
+            case paragraph
+            case heading(Int)
+            case code
+            case quote
+            case listItem(marker: String)
+            case rule
+        }
+
+        let id: Int
+        let kind: Kind
+        let text: AttributedString
+    }
+
+    private var blocks: [Block] {
+        let options = AttributedString.MarkdownParsingOptions(
+            interpretedSyntax: .full,
+            failurePolicy: .returnPartiallyParsedIfPossible
+        )
+        guard let parsed = try? AttributedString(markdown: source, options: options) else {
+            return [Block(id: 0, kind: .paragraph, text: AttributedString(source))]
+        }
+
+        var result: [Block] = []
+        for (intent, range) in parsed.runs[\.presentationIntent] {
+            var text = AttributedString(parsed[range])
+            text.presentationIntent = nil
+
+            guard let intent else {
+                result.append(Block(id: result.count, kind: .paragraph, text: text))
+                continue
+            }
+
+            var kind = Block.Kind.paragraph
+            var ordinal: Int?
+            var ordered = false
+            for component in intent.components {
+                switch component.kind {
+                case .header(let level): kind = .heading(level)
+                case .codeBlock: kind = .code
+                case .blockQuote: kind = .quote
+                case .listItem(let n): ordinal = n
+                case .orderedList: ordered = true
+                case .thematicBreak: kind = .rule
+                default: break
+                }
+            }
+            if case .paragraph = kind, let ordinal {
+                kind = .listItem(marker: ordered ? "\(ordinal)." : "•")
+            }
+            result.append(Block(id: result.count, kind: kind, text: text))
+        }
+        return result
+    }
+
+    @ViewBuilder
+    private func view(for block: Block) -> some View {
+        switch block.kind {
+        case .paragraph:
+            Text(block.text)
+                .textSelection(.enabled)
+        case .heading(let level):
+            Text(block.text)
+                .font(headingFont(level))
+                .textSelection(.enabled)
+                .padding(.top, level <= 2 ? 4 : 0)
+        case .code:
+            ScrollView(.horizontal) {
+                Text(block.text)
+                    .font(.body.monospaced())
+                    .textSelection(.enabled)
+                    .padding(10)
+            }
+            .background(.quaternary.opacity(0.5), in: RoundedRectangle(cornerRadius: 6))
+        case .quote:
+            HStack(alignment: .top, spacing: 10) {
+                RoundedRectangle(cornerRadius: 2)
+                    .fill(.tertiary)
+                    .frame(width: 3)
+                Text(block.text)
+                    .foregroundStyle(.secondary)
+                    .textSelection(.enabled)
+            }
+        case .listItem(let marker):
+            HStack(alignment: .firstTextBaseline, spacing: 8) {
+                Text(marker)
+                    .foregroundStyle(.secondary)
+                    .frame(minWidth: 14, alignment: .trailing)
+                Text(block.text)
+                    .textSelection(.enabled)
+            }
+            .padding(.leading, 4)
+        case .rule:
+            Divider()
+        }
+    }
+
+    private func headingFont(_ level: Int) -> Font {
+        switch level {
+        case 1: return .title.weight(.bold)
+        case 2: return .title2.weight(.semibold)
+        case 3: return .title3.weight(.semibold)
+        default: return .headline
+        }
+    }
+}

ios/KeepMac/MacNoteDetail.swift

@@ -32,10 +32,14 @@ struct MacNoteDetail: View {
             if let n = current {
                 titleField(for: n)
             }
-            TextEditor(text: $text)
-                .font(.body)
-                .textEditorStyle(.plain)
-                .scrollContentBackground(.hidden)
+            if current?.markdown == true {
+                MacMarkdownView(source: text)
+            } else {
+                TextEditor(text: $text)
+                    .font(.body)
+                    .textEditorStyle(.plain)
+                    .scrollContentBackground(.hidden)
+            }
         }
         .padding(12)
         .navigationTitle(current?.displayTitle ?? "New Note")
@@ -50,6 +54,19 @@ struct MacNoteDetail: View {
             .toolbar {
                 if let n = current {
                     ToolbarItemGroup {
+                        Button {
+                            // Markdown and highlight are mutually exclusive,
+                            // matching the web toggles.
+                            let patch: [String: Any] = n.markdown
+                                ? ["markdown": false]
+                                : ["markdown": true, "highlight": false]
+                            Task { await store.update(n.id, patch: patch) }
+                        } label: {
+                            Label(n.markdown ? "Edit Text" : "Preview Markdown",
+                                  systemImage: n.markdown ? "square.and.pencil" : "doc.richtext")
+                        }
+                        .help(n.markdown ? "Back to editing" : "Preview as Markdown")
+
                         Button {
                             Task { await store.togglePin(n) }
                         } label: {