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.go

@@ -25,8 +25,18 @@ type Partition struct {
 	MountPoint string // "" if not mounted
 }
 
+// FormatablePartition is an empty partition on a system disk that can be formatted.
+type FormatablePartition struct {
+	Partition
+	ParentDiskName  string // "sda"
+	ParentDiskPath  string // "/dev/sda"
+	ParentDiskModel string // "Samsung SSD 870"
+	ParentDiskSize  string // "500 GB"
+}
+
 // ScanResult from disk detection.
 type ScanResult struct {
-	AvailableDisks []BlockDevice // Unmounted, non-system disks
-	SystemDisks    []BlockDevice // System/mounted disks (display only)
+	AvailableDisks       []BlockDevice         // Unmounted, non-system disks
+	SystemDisks          []BlockDevice         // System/mounted disks (display only)
+	FormatablePartitions []FormatablePartition // Empty partitions on system disks, safe to format
 }