fix(postinstall): improve error handling and user feedback

Add three critical improvements to post-install phase:

1. Cleanup trap for bind mounts: Ensure /dev, /proc, /sys, /run
   are unmounted even if chroot script fails. Prevents orphaned
   mounts blocking cleanup.

2. Fallocate fallback: Use dd as fallback if fallocate fails
   (can happen on some filesystems or with insufficient space).
   Includes intelligent size conversion and progress display.

3. Progress messages: Add informative echo statements before
   long-running operations (xbps-reconfigure, grub-install,
   grub-mkconfig) so users know the system is working.

Decision: Use trap EXIT instead of manual cleanup to guarantee
execution on both success and error paths. Remove redundant
explicit cleanup call.

Trade-offs: dd fallback is slower but ensures swap file creation
succeeds. Progress messages add noise but significantly improve
UX during multi-minute operations.

src/postinstall.sh

@@ -78,6 +78,7 @@ postinstall_run() {
 
   log_info "Post-install: configuring target system in chroot"
   postinstall_bind_mounts
+  trap postinstall_unbind_mounts EXIT
 
   LUKS_UUID="$luks_uuid" ROOT_UUID="$root_uuid" FS_TYPE="$FS_TYPE" SWAP_SIZE="$SWAP_SIZE" HOSTNAME="$HOSTNAME" \
     chroot "$MOUNT_ROOT" /bin/bash -s <<'EOS'
@@ -117,7 +118,26 @@ if [[ "$SWAP_SIZE" != "0" ]]; then
       btrfs property set /swap compression none || true
     fi
   fi
-  fallocate -l "$SWAP_SIZE" /swap/swapfile
+
+  # Try fallocate first, fall back to dd if it fails
+  if ! fallocate -l "$SWAP_SIZE" /swap/swapfile 2>/dev/null; then
+    echo "fallocate failed, using dd as fallback..."
+    # Extract numeric size and unit for dd
+    local size_num="${SWAP_SIZE%%[^0-9.]*}"
+    local size_unit="${SWAP_SIZE##*[0-9]}"
+    local bs="1M"
+    local count="$size_num"
+
+    # Convert to MB count for dd
+    case "$size_unit" in
+      GiB|G|GB) count=$(awk "BEGIN {print int($size_num * 1024)}") ;;
+      MiB|M|MB) count=$(awk "BEGIN {print int($size_num)}") ;;
+      *) echo "Warning: unknown size unit, assuming MiB"; count="$size_num" ;;
+    esac
+
+    dd if=/dev/zero of=/swap/swapfile bs="$bs" count="$count" status=progress
+  fi
+
   chmod 600 /swap/swapfile
   mkswap /swap/swapfile
   if ! grep -q '^/swap/swapfile' /etc/fstab; then
@@ -168,12 +188,15 @@ else
 fi
 
 # Regenerate initramfs and GRUB configuration.
+echo "Regenerating initramfs for all kernels (this may take a while)..."
 xbps-reconfigure -fa
 
+echo "Installing GRUB bootloader..."
 grub-install --target=x86_64-efi --efi-directory=/boot/efi --bootloader-id=Void --recheck
+
+echo "Generating GRUB configuration..."
 grub-mkconfig -o /boot/grub/grub.cfg
 EOS
 
-  postinstall_unbind_mounts
   log_info "Post-install complete."
 }