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/config/config.go

@@ -12,6 +12,7 @@ import (
 
 type Config struct {
 	AppName          string
+	AppVersion       string
 	Environment      string
 	Addr             string
 	BaseURL          string
@@ -54,6 +55,7 @@ type SettingsDefaults struct {
 func Load() (Config, error) {
 	cfg := Config{
 		AppName:          envString("WARPBOX_APP_NAME", "warpbox.dev"),
+		AppVersion:       envString("APP_VERSION", "dev"),
 		Environment:      envString("WARPBOX_ENV", "development"),
 		Addr:             envString("WARPBOX_ADDR", ":8080"),
 		BaseURL:          strings.TrimRight(envString("WARPBOX_BASE_URL", "http://localhost:8080"), "/"),