Enhance Raspberry Pi OS image handling and dashboard UI

Update the API to support streaming and decompressing of golden images in .img.xz and .img.gz formats on the fly. Modify the cloud-init build process to compress images to .img.xz for size reduction. Revise the dashboard templates to set 'desktop' as the default variant for Raspberry Pi OS builds, improving user experience and clarity in options. Update related scripts to ensure compatibility with the new image handling features.

emmc-provisioning/host/build-cloudinit-image.sh

@@ -165,6 +165,21 @@ write_status "finalizing" "Copying image to cloud-init images…" "" ""
 mkdir -p "$OUTPUT_DIR"
 cp "$IMG_FILE" "$OUT_PATH"
 
+# Compress to .img.xz to reduce size (deploy supports decompress on the fly)
+write_status "finalizing" "Compressing image (xz)…" "" ""
+OUT_XZ="${OUT_PATH}.xz"
+# -T 0 = use all cores; fallback to -T 1 if old xz
+if ! xz -T 0 -z -k -f "$OUT_PATH" 2>/dev/null; then
+    xz -T 1 -z -k -f "$OUT_PATH" 2>/dev/null || true
+fi
+if [[ -f "$OUT_XZ" ]]; then
+    rm -f "$OUT_PATH"
+    OUT_NAME="${OUT_NAME}.xz"
+    OUT_PATH="$OUT_XZ"
+else
+    write_status "finalizing" "Compression skipped (xz failed), keeping raw image." "" ""
+fi
+
 if [[ "$SET_AS_GOLDEN" == "1" ]]; then
     rm -f "$GOLDEN_IMAGE"
     ln -sf "$OUT_PATH" "$GOLDEN_IMAGE"