ci: package built extension as a downloadable artifact for sideloading

Adds Package + upload-artifact steps so every CI run produces
podkey-extension.zip (manifest + src + popup + icons), loadable via
chrome://extensions -> Load unpacked. Answers 'does CI produce a
sideloadable extension'.

Co-Authored-By: jjohare <github@thedreamlab.uk>

Author: jjohare

.github/workflows/ci.yml

@@ -33,3 +33,15 @@ jobs:
 
       - name: Lint
         run: npm run lint
+
+      - name: Package extension (sideloadable unpacked zip)
+        run: |
+          mkdir -p dist
+          zip -r dist/podkey-extension.zip manifest.json src popup $([ -d icons ] && echo icons) -x '*.test.js'
+
+      - name: Upload extension artifact
+        uses: actions/upload-artifact@v4
+        with:
+          name: podkey-extension
+          path: dist/podkey-extension.zip
+          if-no-files-found: error