The board context menu's Flash item and FlashModal gain a second
backend: a Web Serial flasher the pro overlay can install via
lib/proWebFlash.ts (installWebFlashImpl). Without an overlay the seam
is inert — web builds keep the desktop-only behavior. esptool-js is
declared here because the overlay has no package.json of its own; OSS
code never imports it (verified absent from the vite bundle).
David Montero Crespo