Add markdown formatting hotkeys to the editor

Ctrl/Cmd+B/I/E wrap the selection in bold / italic / inline-code markers,
Ctrl/Cmd+Shift+X in strikethrough, and Ctrl/Cmd+K as a link (with the url
placeholder selected). Pressing the same key on already-wrapped text
removes the markers. Applied to the live selection via the TextEdit
cursor range, with the caret/selection restored around the edit.

Documented in the built-in cheatsheet and the README.

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

src/help.rs

@@ -54,6 +54,24 @@ fn cheatsheet_body(ui: &mut egui::Ui) {
         ],
     );
 
+    section(ui, "Formatting hotkeys");
+    syntax(
+        ui,
+        "cs-keys",
+        &[
+            ("Ctrl/Cmd + B", "Bold the selection (**…**)."),
+            ("Ctrl/Cmd + I", "Italicise the selection (*…*)."),
+            ("Ctrl/Cmd + E", "Inline code (`…`)."),
+            ("Ctrl/Cmd + Shift + X", "Strikethrough (~~…~~)."),
+            ("Ctrl/Cmd + K", "Wrap as a link and select the url to replace."),
+        ],
+    );
+    note(
+        ui,
+        "Applied to the selected text (or the caret, for an empty selection). \
+         Pressing the same key on already-wrapped text removes the markers.",
+    );
+
     section(ui, "Paragraphs & breaks");
     syntax(
         ui,