fix: Implement proper block-level Borg backups

BREAKING CHANGE: Borg backups now store raw block devices instead of files

Changes:
- LV→Borg: Stores snapshot as raw block device (.img file) in Borg
- VG→Borg: Stores each LV as separate .img files in Borg repository
- No more file-level mounting - preserves exact block-level state
- Uses dd | borg create --stdin-name for LV backups
- Creates temporary .img files for VG backups
- Maintains all filesystem metadata, boot sectors, etc.
- Better deduplication for similar block patterns

Benefits:
- Exact block-level restoration possible
- Preserves all filesystem metadata
- Better suited for system/boot volume backups
- Still gets Borg's compression, deduplication, encryption
- Clear difference between LV and VG modes

Now LV→Borg and VG→Borg have distinct, useful purposes:
- LV→Borg: Single logical volume as one block device
- VG→Borg: All logical volumes as separate block devices

README.md

@@ -18,8 +18,8 @@ No complex migration logic, no fancy features that can break. Just reliable back
 - **LV → LV**: Update existing logical volume backups
 - **LV → Raw**: Create fresh backups on raw devices
 - **VG → Raw**: Clone entire volume groups with LVM metadata
-- **LV → Borg**: Backup logical volume to Borg repository (deduplicated, compressed, encrypted)
-- **VG → Borg**: Backup entire volume group to Borg repository
+- **LV → Borg**: Backup logical volume **block device** to Borg repository (preserves exact block-level state)
+- **VG → Borg**: Backup **all block devices** from volume group to Borg repository
 
 ### Two Interfaces
 
@@ -55,15 +55,19 @@ sudo ./enhanced_simple_backup.sh vg-to-raw internal-vg /dev/sdb
 sudo python3 simple_backup_gui.py
 ```
 
-**Borg repository backups:**
+**Borg repository backups (block-level):**
 ```bash
-# Create new Borg repo and backup LV
+# Create new Borg repo and backup LV as block device
 sudo ./enhanced_simple_backup.sh lv-to-borg /dev/internal-vg/root /path/to/borg/repo --new-repo --encryption repokey --passphrase mypass
 
-# Backup entire VG to existing repo
+# Backup entire VG as multiple block devices to existing repo
 sudo ./enhanced_simple_backup.sh vg-to-borg internal-vg /path/to/borg/repo --passphrase mypass
 ```
 
+**Key Difference:**
+- **LV → Borg**: Stores the raw snapshot as `{lv_name}.img` in Borg
+- **VG → Borg**: Stores all LVs as separate `.img` files (`root.img`, `home.img`, etc.)
+
 ## Files
 
 ### Core System