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/web/storage_handlers.go

@@ -214,10 +214,11 @@ func (s *Server) storageScanAPIHandler(w http.ResponseWriter, r *http.Request) {
 		s.logger.Printf("[DEBUG] [web] storageScan: found %d available disks, %d system disks", len(result.AvailableDisks), len(result.SystemDisks))
 	}
 	jsonResponse(w, map[string]interface{}{
-		"ok":             true,
-		"available":      result.AvailableDisks,
-		"system":         result.SystemDisks,
-		"available_count": len(result.AvailableDisks),
+		"ok":                    true,
+		"available":             result.AvailableDisks,
+		"system":                result.SystemDisks,
+		"available_count":       len(result.AvailableDisks),
+		"formatable_partitions": result.FormatablePartitions,
 	})
 }