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.

Dockerfile

@@ -16,12 +16,15 @@ RUN CGO_ENABLED=0 GOOS=linux go build \
 
 FROM alpine:3.22
 
+ARG APP_VERSION=dev
+
 RUN apk add --no-cache ca-certificates ffmpeg wget
 
 ENV WARPBOX_ADDR=:8080 \
     WARPBOX_DATA_DIR=/data \
     WARPBOX_STATIC_DIR=/app/static \
-    WARPBOX_TEMPLATE_DIR=/app/templates
+    WARPBOX_TEMPLATE_DIR=/app/templates \
+    APP_VERSION=${APP_VERSION}
 
 WORKDIR /app