feat: add web-ext tooling for Firefox signing

Add package.json with web-ext dev dependency and npm scripts for
building, linting, signing, and running the Firefox extension.
Signing produces a self-hosted .xpi that persists across restarts
without needing the Mozilla store.

https://claude.ai/code/session_01Dvgwe7XMoSxnWXkih8p1Cw

.gitignore

@@ -3,4 +3,7 @@ node_modules/
 *.crx
 *.pem
 *.zip
+*.xpi
 dist/
+.env
+web-ext-artifacts/