Add a Help menu with a built-in Markdown cheatsheet

Mirrors the logrs cheatsheet: a scrollable reference window covering the
markdown md-manuscript renders on export, plus the manuscript-specific
conventions (# Title:/# Slug: metadata, the draft-marker header, and
comments). Opened from Help ▸ Markdown cheatsheet in the menu bar.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

README.md

@@ -181,3 +181,7 @@ Headings, paragraphs, **bold**, *italic*, ***both***, `inline code`, fenced/inde
 code blocks (monospace, whitespace preserved), ~~strikethrough~~, ordered and
 unordered (nested) lists, block quotes, horizontal rules, and links. Images are
 rendered as a text placeholder (not embedded).
+
+A quick reference for all of this — plus the chapter metadata, draft-marker
+header, and comment conventions — is built in: open **Help ▸ Markdown cheatsheet**
+from the menu bar.

src/app.rs

@@ -63,6 +63,8 @@ pub struct App {
     settings_test_rx: Option<std::sync::mpsc::Receiver<Result<usize, String>>>,
     /// Result line for the settings window's "Test connection" button.
     settings_test_status: String,
+    /// Whether the markdown cheatsheet window is open.
+    show_cheatsheet: bool,
 }
 
 impl App {
@@ -96,6 +98,7 @@ impl App {
             show_settings: false,
             settings_test_rx: None,
             settings_test_status: String::new(),
+            show_cheatsheet: false,
         };
         app.open_workspace();
         app
@@ -969,6 +972,12 @@ impl App {
                         self.show_settings = true;
                     }
                 });
+                ui.menu_button("Help", |ui| {
+                    if ui.button("📝 Markdown cheatsheet").clicked() {
+                        ui.close_menu();
+                        self.show_cheatsheet = true;
+                    }
+                });
             });
         });
     }
@@ -1411,6 +1420,10 @@ impl eframe::App for App {
             self.settings_window(ctx);
         }
 
+        if self.show_cheatsheet {
+            crate::help::cheatsheet_window(ctx, &mut self.show_cheatsheet);
+        }
+
         if self.pending_repo.is_some() {
             self.repo_prompt(ctx);
         }

src/help.rs

@@ -0,0 +1,169 @@
+//! In-app **Markdown cheatsheet** window: a quick reference for the markdown
+//! md-manuscript understands, plus the manuscript-specific conventions (chapter
+//! metadata, the draft-marker header, and comments) applied at export time.
+//!
+//! Pure rendering — the caller owns the `open` flag so the window's ✕ works via
+//! [`egui::Window::open`].
+
+use eframe::egui;
+
+/// The Markdown syntax cheatsheet window.
+pub fn cheatsheet_window(ctx: &egui::Context, open: &mut bool) {
+    egui::Window::new("📝 Markdown Cheatsheet")
+        .open(open)
+        .resizable(true)
+        .default_width(560.0)
+        .default_height(560.0)
+        .show(ctx, |ui| {
+            egui::ScrollArea::vertical().show(ui, cheatsheet_body);
+        });
+}
+
+fn cheatsheet_body(ui: &mut egui::Ui) {
+    intro(
+        ui,
+        "The markdown md-manuscript renders on export. Left column is what you type.",
+    );
+
+    section(ui, "Headings");
+    syntax(
+        ui,
+        "cs-headings",
+        &[
+            ("# Heading", "A top-level heading (nested under the chapter title on export)."),
+            ("## Sub-heading", "A second-level heading; deeper levels use more #."),
+        ],
+    );
+    note(
+        ui,
+        "On export, every heading is pushed down one level so it sits beneath the \
+         chapter's own title. See “Chapter metadata” below for setting that title.",
+    );
+
+    section(ui, "Emphasis & inline");
+    syntax(
+        ui,
+        "cs-inline",
+        &[
+            ("**bold**", "Bold text."),
+            ("*italic*", "Italic text (also _italic_)."),
+            ("***bold italic***", "Both at once."),
+            ("~~strike~~", "Strikethrough."),
+            ("`code`", "Inline monospaced code."),
+            ("[text](https://url)", "A hyperlink."),
+        ],
+    );
+
+    section(ui, "Paragraphs & breaks");
+    syntax(
+        ui,
+        "cs-para",
+        &[
+            ("blank line", "Separates paragraphs."),
+            ("--- (on its own line)", "A horizontal rule — handy as a scene break."),
+        ],
+    );
+
+    section(ui, "Lists");
+    syntax(
+        ui,
+        "cs-lists",
+        &[
+            ("- item", "An unordered list item (also * item or + item)."),
+            ("  - nested", "Indent two spaces to nest a sub-item."),
+            ("1. item", "An ordered (numbered) list item."),
+            ("> quote", "A blockquote."),
+        ],
+    );
+
+    section(ui, "Code blocks");
+    syntax(
+        ui,
+        "cs-code",
+        &[
+            ("```", "Open and close a fenced code block on their own lines."),
+            ("    four-space indent", "An indented code block."),
+        ],
+    );
+
+    section(ui, "Chapter metadata");
+    syntax(
+        ui,
+        "cs-meta",
+        &[
+            ("# Title: Chapter Name", "Sets this file's chapter heading on export."),
+            ("# Slug: a caption", "A caption shown beneath the chapter title."),
+        ],
+    );
+    note(
+        ui,
+        "These lines are lifted out and removed from the exported prose. Without a \
+         # Title:, the chapter falls back to a manual override (the title field) or \
+         its position number.",
+    );
+
+    section(ui, "Editorial header & comments");
+    syntax(
+        ui,
+        "cs-header",
+        &[
+            ("### Rough Draft:", "The draft marker — everything above it is your working header."),
+            ("<!-- note -->", "A comment; dropped on export (may span multiple lines)."),
+        ],
+    );
+    note(
+        ui,
+        "The draft marker text is configurable in the top bar. Everything above the \
+         marker line is treated as notes and left out of the export; leave the marker \
+         blank to export the whole file. Comments are removed everywhere.",
+    );
+
+    section(ui, "Grammar & spelling");
+    body(
+        ui,
+        "Press ✓ Check (or the Grammar panel) to run the current file through your \
+         LanguageTool server — configure it in Settings ▸ LanguageTool. Spelling issues \
+         are underlined in red, grammar/style in blue.",
+    );
+}
+
+// --- small rendering helpers --------------------------------------------------
+
+fn intro(ui: &mut egui::Ui, text: &str) {
+    ui.label(egui::RichText::new(text).italics().weak());
+    ui.add_space(4.0);
+}
+
+fn section(ui: &mut egui::Ui, title: &str) {
+    ui.add_space(8.0);
+    ui.heading(title);
+    ui.separator();
+}
+
+fn body(ui: &mut egui::Ui, text: &str) {
+    ui.label(text);
+}
+
+fn note(ui: &mut egui::Ui, text: &str) {
+    ui.add_space(2.0);
+    ui.label(egui::RichText::new(format!("ⓘ {text}")).small().weak());
+}
+
+/// A two-column grid of `syntax/meaning` rows (syntax in a highlighted monospace).
+fn syntax(ui: &mut egui::Ui, id: &str, rows: &[(&str, &str)]) {
+    egui::Grid::new(id)
+        .num_columns(2)
+        .striped(true)
+        .spacing([18.0, 4.0])
+        .show(ui, |ui| {
+            for (s, v) in rows {
+                ui.label(
+                    egui::RichText::new(*s)
+                        .monospace()
+                        .color(egui::Color32::from_rgb(120, 170, 120)),
+                );
+                ui.label(*v);
+                ui.end_row();
+            }
+        });
+}

src/main.rs

@@ -5,6 +5,7 @@
 mod app;
 mod config;
 mod gitsync;
+mod help;
 mod langtool;
 mod odt;
 mod order;