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/safety_other.go

@@ -8,6 +8,10 @@ func IsSystemDisk(devicePath string) (bool, error) {
 	return false, fmt.Errorf("storage init is only supported on Linux")
 }
 
+func IsSystemPartition(partitionPath string) (bool, error) {
+	return false, fmt.Errorf("storage init is only supported on Linux")
+}
+
 func IsDeviceMounted(devicePath string) (bool, error) {
 	return false, fmt.Errorf("storage init is only supported on Linux")
 }