docs: add BUILD.md for Mozilla reviewer build instructions

BUILD.md

@@ -0,0 +1,18 @@
+# Building inØsight
+
+## Prerequisites
+- Node.js (v20+ or v22+)
+- npm (v10+)
+
+## Step-by-Step Build Instructions
+1. Install dependencies:
+   ```bash
+   npm install
+   ```
+
+2. Build the Firefox extension:
+   ```bash
+   npm run build:firefox
+   ```
+
+3. The generated add-on files will be in `dist/firefox/`.