feat: format empty partitions on system disk (v0.32.6)

Detect and offer to format empty (no filesystem) partitions on the system
disk. Adds IsSystemPartition() for granular per-partition safety checks
instead of blocking the entire system disk. Init wizard shows formatable
partitions with appropriate warnings. Add felhotest demo node to docs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/storage/format_linux.go

@@ -61,12 +61,25 @@ func FormatAndMount(req FormatRequest, progress chan<- FormatProgress) (string,
 		return "", fail("validating", "Az eszköz nem létezik: "+req.DevicePath, err)
 	}
 
-	isSystem, err := IsSystemDisk(req.DevicePath)
-	if err != nil {
-		return "", fail("validating", "Rendszermeghajtó ellenőrzése sikertelen", err)
-	}
-	if isSystem {
-		return "", fail("validating", "Ez a rendszermeghajtó — nem formázható!", fmt.Errorf("device is system disk"))
+	if req.CreatePartition {
+		// Whole-disk operation: block if any partition on this disk is a system partition.
+		isSystem, err := IsSystemDisk(req.DevicePath)
+		if err != nil {
+			return "", fail("validating", "Rendszermeghajtó ellenőrzése sikertelen", err)
+		}
+		if isSystem {
+			return "", fail("validating", "Ez a rendszermeghajtó — nem formázható!", fmt.Errorf("device is system disk"))
+		}
+	} else {
+		// Partition-only operation: block only if THIS specific partition is a system partition.
+		// Allows formatting empty partitions on the system disk (e.g., dedicated data partition).
+		isSysPart, err := IsSystemPartition(req.DevicePath)
+		if err != nil {
+			return "", fail("validating", "Rendszerpartíció ellenőrzése sikertelen", err)
+		}
+		if isSysPart {
+			return "", fail("validating", "Ez rendszerpartíció — nem formázható!", fmt.Errorf("partition is a system partition"))
+		}
 	}
 
 	mounted, err := IsDeviceMounted(req.DevicePath)