fix(workbench): Keep a way out of Distraction Free Mode on Windows (#47)

Root cause: on Windows the window is frameless and the main menu lives in the custom title bar; Distraction Free Mode rendered a bare title bar without it, the toggle has no default shortcut (as in IDEA) and the mode persists across restarts, leaving no way out.
Fix (IDEA): the bare title bar keeps the main menu when there is no native menu bar; a lone Alt opens the main menu; entering the mode (or restoring it) shows "To exit Distraction Free Mode, use View | Appearance … or Find Action" with an Exit action, in its own notification group.
Siblings covered: Distraction Free + Full Screen on Windows; other chrome-hiding settings checked (unaffected).
Verified in-app on macOS (hint, Exit action, single Don't Show Again). Windows-only parts (☰ in bare title bar, lone Alt) not verified on Windows.

Claude-Session: https://claude.ai/code/session_01VUYsdXA2nkF5v6zsC93dpn

src/workbench/MainMenu.test.ts

@@ -1,5 +1,56 @@
 import { describe, expect, it } from "vitest";
-import { menuItems } from "./MainMenu";
+import { menuItems, onLoneAlt } from "./MainMenu";
+
+describe("lone Alt opens the main menu", () => {
+  const key = (type: string, k: string, init: KeyboardEventInit = {}) =>
+    window.dispatchEvent(new KeyboardEvent(type, { key: k, bubbles: true, cancelable: true, ...init }));
+  it.each<[string, () => void, number]>([
+    ["Alt alone", () => (key("keydown", "Alt", { altKey: true }), key("keyup", "Alt")), 1],
+    [
+      "Alt+1",
+      () => (
+        key("keydown", "Alt", { altKey: true }),
+        key("keydown", "1", { altKey: true }),
+        key("keyup", "1"),
+        key("keyup", "Alt")
+      ),
+      0,
+    ],
+    [
+      "Ctrl+Alt",
+      () => (
+        key("keydown", "Control", { ctrlKey: true }),
+        key("keydown", "Alt", { ctrlKey: true, altKey: true }),
+        key("keyup", "Alt")
+      ),
+      0,
+    ],
+    [
+      "Alt+click",
+      () => (
+        key("keydown", "Alt", { altKey: true }),
+        window.dispatchEvent(new Event("pointerdown")),
+        key("keyup", "Alt")
+      ),
+      0,
+    ],
+    [
+      "held Alt (repeat)",
+      () => (
+        key("keydown", "Alt", { altKey: true }),
+        key("keydown", "Alt", { altKey: true, repeat: true }),
+        key("keyup", "Alt")
+      ),
+      0,
+    ],
+  ])("%s", (_, press, expected) => {
+    let opened = 0;
+    const off = onLoneAlt(window, () => void opened++);
+    press();
+    off();
+    expect(opened).toBe(expected);
+  });
+});
 
 describe("main menu model", () => {
   it("maps menu nodes to menu items, localising spec labels only", () => {
@@ -11,7 +62,13 @@ describe("main menu model", () => {
         id: "openRecent",
         label: "Open Recent",
         children: [
-          { kind: "action", id: "file.openRecentProject", label: "openidea  —  ~/openidea", hint: null, arg: "/p/openidea" },
+          {
+            kind: "action",
+            id: "file.openRecentProject",
+            label: "openidea  —  ~/openidea",
+            hint: null,
+            arg: "/p/openidea",
+          },
           { kind: "placeholder", id: "recent.none", label: "No Recent Projects" },
         ],
       },