add in ppmpredictor - would usually usually webpack this but for this just adding it

Author: willwade

browser/index.html

@@ -19,7 +19,7 @@
     <script type="importmap">
     {
         "imports": {
-            "@willwade/ppmpredictor": "../node_modules/@willwade/ppmpredictor/dist/ppmpredictor.esm.js"
+            "@willwade/ppmpredictor": "./vendor/ppmpredictor.esm.js"
         }
     }
     </script>

browser/vendor/README.md

@@ -0,0 +1,9 @@
+This directory contains vendored browser assets that must be served in static deployments.
+
+Current files:
+- `ppmpredictor.esm.js` copied from `node_modules/@willwade/ppmpredictor/dist/ppmpredictor.esm.js`
+
+Update flow:
+1. Run `npm ci` in repo root.
+2. Copy the latest dist file into this directory.
+3. Commit both the vendored file and this README update (if version/source changes).