docs(live-preview): notes for static file:// preview of non-project html

Records a verified Electron prototype that previews html files outside the
project as static file:// pages in a hardened <webview> with reload on save,
instead of the "Preview Unavailable" page. Includes the security findings
(Electron file:// pages can read local files unless the guest session filters
requests), the rolled back Electron main-process code, the verification steps
and the full patch for this repo. Adds JSDoc pointers to the notes at the
relevant functions. No behaviour change.

Author: abose