feat: add application versioning support to backend and UI

- Introduce APP_VERSION build argument and environment variable in Dockerfile.
- Load AppVersion from environment variables in the configuration loader.
- Pass the application version to the HTML renderer and expose it to templates via PageData.
- Update tests to verify the version is correctly rendered in the footer.

backend/templates/layouts/base.html

@@ -60,7 +60,7 @@
   </main>
 
   <footer class="site-footer">
-    <span>{{.AppName}} · {{.CurrentYear}} · self-hosted</span>
+    <span>{{.AppName}} · {{.AppVersion}} · {{.CurrentYear}} · self-hosted</span>
     <label class="theme-picker">
       <span>Theme</span>
       <select data-theme-select aria-label="Site theme">