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/libs/httpserver/server.go

@@ -13,7 +13,7 @@ import (
 )
 
 func New(cfg config.Config, logger *slog.Logger) (*http.Server, error) {
-	renderer, err := web.NewRenderer(cfg.TemplateDir, cfg.AppName, cfg.BaseURL)
+	renderer, err := web.NewRenderer(cfg.TemplateDir, cfg.AppName, cfg.AppVersion, cfg.BaseURL)
 	if err != nil {
 		return nil, err
 	}