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

web-ext-config.js

@@ -0,0 +1,34 @@
+/**
+ * web-ext configuration for Firefox extension development.
+ *
+ * For signing, you need a Mozilla API key and secret.
+ * Get them at: https://addons.mozilla.org/developers/addon/api/key/
+ *
+ * Set these environment variables before running `npm run sign:firefox`:
+ *   export WEB_EXT_API_KEY="user:12345:678"
+ *   export WEB_EXT_API_SECRET="your-secret-here"
+ *
+ * Or create a .env file (git-ignored) and source it:
+ *   source .env && npm run sign:firefox
+ */
+
+module.exports = {
+  sourceDir: 'dist/firefox',
+  artifactsDir: 'dist/firefox-signed',
+  ignoreFiles: [
+    'generate-icons.html',
+  ],
+  build: {
+    overwriteDest: true,
+  },
+  run: {
+    startUrl: ['https://claude.ai/'],
+    // Firefox Developer Edition or Nightly recommended for MV3
+    // firefox: '/path/to/firefox-developer-edition',
+  },
+  sign: {
+    channel: 'unlisted',
+    // API credentials come from environment variables:
+    //   WEB_EXT_API_KEY and WEB_EXT_API_SECRET
+  },
+};