Improve NVMe serial parsing and restructure MergerFS display

- Fix NVMe serial number parsing to handle whitespace variations
- Move mount point to MergerFS header, remove drive count
- Restructure data drives to same level as parity with Data_1, Data_2 labels
- Remove "Total:" label from pool usage line
- Update parity to use closing tree symbol as last item

agent/src/collectors/disk.rs

@@ -457,11 +457,15 @@ impl DiskCollector {
                 }
             }
             
-            // Serial number parsing
-            if line.starts_with("Serial Number:") {
+            // Serial number parsing (both SATA and NVMe)
+            if line.contains("Serial Number:") {
                 if let Some(serial_part) = line.split("Serial Number:").nth(1) {
-                    if let Some(serial_str) = serial_part.split_whitespace().next() {
-                        serial_number = Some(serial_str.to_string());
+                    let serial_str = serial_part.trim();
+                    if !serial_str.is_empty() {
+                        // Take first whitespace-separated token
+                        if let Some(serial) = serial_str.split_whitespace().next() {
+                            serial_number = Some(serial.to_string());
+                        }
                     }
                 }
             }