Fix CI: add mkdocs-static-i18n + pin mkdocs<2

The i18n plugin was added to mkdocs.yml but never installed in CI.
Meanwhile MkDocs 2.0 dropped the plugin system entirely.
Pin mkdocs to 1.x and mkdocs-material to 9.x to restore builds.

Made-with: Cursor

Author: SyntaxArchmage

.github/workflows/deploy-docs.yml

@@ -21,7 +21,7 @@ jobs:
           python-version: '3.12'
 
       - name: Install dependencies
-        run: pip install mkdocs mkdocs-material
+        run: pip install "mkdocs>=1.6,<2" "mkdocs-material>=9,<10" mkdocs-static-i18n
 
       - name: Build and deploy
         run: mkdocs gh-deploy --force