feat: lazy load ninja-keys + HTML Invoker Commands API

- Lazy load ninja-keys only on CMD+K press (0 requests on initial load)
- Use esm.sh bundled module (3 requests vs ~81 previously)
- Add esm.sh to CSP whitelist
- Implement HTML Invoker Commands API for modals:
  - commandfor="modal-id" + command="show-modal" for opening
  - commandfor="modal-id" + command="close" for closing
  - Removes need for onclick handlers on modal buttons
- Refactor index.html into layout partials (head, body-scripts)
- Add comprehensive tests for both features

templates/partials/widgets/info-button.html

@@ -1,9 +1,11 @@
 {{define "info-button"}}
 <!-- Info Button (Bottom Left) -->
+<!-- Uses HTML Invoker Commands API: commandfor + command="show-modal" -->
 <button id="info-button" class="info-button no-print has-tooltip"
         aria-label="{{.UI.Widgets.Info.AriaLabel}}"
         data-tooltip="{{.UI.Widgets.Info.Tooltip}}"
-        onclick="document.getElementById('info-modal').showModal()">
+        commandfor="info-modal"
+        command="show-modal">
     <iconify-icon icon="mdi:information-outline"></iconify-icon>
 </button>
 {{end}}