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>

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);
         }