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/scan_linux.go

@@ -164,6 +164,58 @@ func getSystemDiskNames() map[string]bool {
 	return systemDisks
 }
 
+// getSystemPartitionPaths returns the set of partition device paths (e.g., "/dev/sda3")
+// that are system partitions (/, /boot, /boot/efi, swap).
+// Unlike getSystemDiskNames() which returns parent disk names, this returns the actual
+// partition paths for granular checks.
+func getSystemPartitionPaths() map[string]bool {
+	sysPartitions := map[string]bool{}
+
+	fstabPath := "/host-fstab"
+	if _, err := os.Stat(fstabPath); err != nil {
+		fstabPath = "/etc/fstab"
+	}
+
+	data, err := os.ReadFile(fstabPath)
+	if err != nil {
+		return sysPartitions
+	}
+
+	systemMounts := map[string]bool{"/": true, "/boot": true, "/boot/efi": true}
+
+	for _, line := range strings.Split(string(data), "\n") {
+		line = strings.TrimSpace(line)
+		if line == "" || strings.HasPrefix(line, "#") {
+			continue
+		}
+		fields := strings.Fields(line)
+		if len(fields) < 3 {
+			continue
+		}
+		source := fields[0]
+		mountPoint := fields[1]
+		fsType := fields[2]
+
+		if !systemMounts[mountPoint] && fsType != "swap" {
+			continue
+		}
+
+		if strings.HasPrefix(source, "UUID=") {
+			uuid := strings.TrimPrefix(source, "UUID=")
+			if out, err := exec.Command("blkid", "-U", uuid).Output(); err == nil {
+				devPath := strings.TrimSpace(string(out))
+				if devPath != "" {
+					sysPartitions[devPath] = true
+				}
+			}
+		} else if strings.HasPrefix(source, "/dev/") {
+			sysPartitions[source] = true
+		}
+	}
+
+	return sysPartitions
+}
+
 // partitionToParentDisk extracts the parent disk name from a partition device path.
 // "/dev/sda2" → "sda", "/dev/nvme0n1p2" → "nvme0n1", "/dev/mmcblk0p1" → "mmcblk0"
 func partitionToParentDisk(devPath string) string {
@@ -344,8 +396,31 @@ func ScanDisks(logger *log.Logger, debug bool) (*ScanResult, error) {
 	enrichWithBlkid(result.AvailableDisks, logger, debug)
 	enrichWithBlkid(result.SystemDisks, logger, debug)
 
+	// Detect formatable partitions on system disks:
+	// empty (no filesystem), unmounted, and NOT a system partition.
+	sysPartitionPaths := getSystemPartitionPaths()
+	for _, sysDisk := range result.SystemDisks {
+		for _, part := range sysDisk.Partitions {
+			if part.FSType != "" || part.MountPoint != "" {
+				continue
+			}
+			if sysPartitionPaths[part.Path] {
+				continue
+			}
+			result.FormatablePartitions = append(result.FormatablePartitions, FormatablePartition{
+				Partition:       part,
+				ParentDiskName:  sysDisk.Name,
+				ParentDiskPath:  sysDisk.Path,
+				ParentDiskModel: sysDisk.Model,
+				ParentDiskSize:  sysDisk.Size,
+			})
+			dbg("formatable partition found: %s on system disk %s", part.Path, sysDisk.Name)
+		}
+	}
+
 	if logger != nil {
-		logger.Printf("[INFO] [storage] Found %d disks", len(result.AvailableDisks)+len(result.SystemDisks))
+		logger.Printf("[INFO] [storage] Found %d disks, %d formatable partitions",
+			len(result.AvailableDisks)+len(result.SystemDisks), len(result.FormatablePartitions))
 	}
 	dbg("disk scan completed in %s", time.Since(scanStart).Round(time.Millisecond))