R-889: every disk percent is df's (used / (used + available)); the tile says Rendszer, not Rendszer (/) (09 decision 138)

The box divided used by the whole filesystem, so the 5 % reserved for root made a full disk read ~5 points low
(61.8 of 68.7 GB = 90 % where df said 95 %) and the fill alarms fire late. One function, DFUsedPercent, now serves
GetDiskUsage, readDiskUsage, the recovery-unit headroom projection and the deploy page's free percent. Tests use
numbers measured on demo-hp 9201 (/mnt/sys_drive: old 21.5 %, df 23 %); a source scan refuses a percent divided by
the whole filesystem (red-proved by putting the old line back). The label measured the docker data volume, not /.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

controller/internal/web/handlers.go

@@ -569,7 +569,7 @@ func (s *Server) deployHandler(w http.ResponseWriter, r *http.Request, name stri
 		if di := system.GetDiskUsage(sp.Path); di != nil {
 			dp.FreeHuman = formatFreeSpace(di.AvailGB)
 			if di.TotalGB > 0 {
-				dp.FreePercent = di.AvailGB / di.TotalGB * 100
+				dp.FreePercent = 100 - system.DFUsedPercent(di.UsedGB, di.AvailGB) // R-889: df's number
 			}
 		}
 		deployPaths = append(deployPaths, dp)