feat(cms): keyboard shortcuts — Ctrl+S save, Ctrl+P publish, ? help

- Ctrl/Cmd+S saves (native browser save dialog suppressed)
- Ctrl/Cmd+P publishes; Ctrl/Cmd+Shift+V opens the Versions panel
- '?' toggles a shortcuts overlay (Esc/click closes)
- plain typing in inputs never triggers actions (modifiers required;
  '?' only outside editing targets)
- jsdom regression tests run the real client script with dispatched
  KeyboardEvents; the client is evaluated once per suite because each
  eval would stack another keydown listener on the shared document

Closes MITHOME-75

scripts/test-content-editor-serializer.js

@@ -45,6 +45,8 @@ const document = {
   getElementById: id => id === 'page-data' ? { textContent: JSON.stringify(fixture) } : {},
   querySelectorAll: selector => selector === '[data-path]' ? fields : [],
   querySelector: () => null,
+  // Keyboard-shortcut binding in the client script — not under test here.
+  addEventListener: () => {},
 };
 const browserContext = { document, console, setTimeout, fetch: async () => ({ json: async () => ({ ok: true }) }) };
 vm.createContext(browserContext);