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/system/mounts_linux.go

@@ -82,7 +82,7 @@ func GetDiskUsage(path string) *DiskUsageInfo {
 		AvailGB: float64(avail) / float64(gb),
 	}
 	if total > 0 {
-		info.UsedPercent = float64(used) / float64(total) * 100
+		info.UsedPercent = DFUsedPercent(float64(used), float64(avail)) // R-889: df's number
 	}
 	info.TotalHuman = formatGB(info.TotalGB)
 	info.UsedHuman = formatGB(info.UsedGB)