Fix Firefox MV3 build: background.scripts with bundled classic script ...

Firefox MV3 does not support background.service_worker or type:"module",
so loading the root manifest failed with "background.service_worker is
currently disabled". The Firefox build now bundles src/api.js +
src/background.js into a single classic script (stripping import/export)
via tools/bundle-background.js and switches the manifest to
background.scripts. The Chrome build keeps service_worker + type:module.
Update README to load build/firefox (or the dist zip) in Firefox.

Co-Authored-By: Claude <noreply@anthropic.com>