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

.env.example

@@ -0,0 +1,8 @@
+# Mozilla Add-on API credentials for Firefox extension signing
+# Get yours at: https://addons.mozilla.org/developers/addon/api/key/
+#
+# Copy this file to .env and fill in your values:
+#   cp .env.example .env
+
+WEB_EXT_API_KEY="user:12345:678"
+WEB_EXT_API_SECRET="your-secret-here"