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/backup/recovery_unit.go

@@ -416,7 +416,7 @@ func (m *Manager) floorVerdict(u *UnitSpace, estGiB float64) (*UnitSpace, floorR
 		availAfter := u.AvailGB - estGiB
 		usedAfter := u.UsedPercent
 		if u.TotalGB > 0 {
-			usedAfter = (u.UsedGB + estGiB) / u.TotalGB * 100
+			usedAfter = system.DFUsedPercent(u.UsedGB+estGiB, u.AvailGB-estGiB) // R-889: df's number after the write
 		}
 		if availAfter < FloorFreeGiB || usedAfter >= FloorUsedPercent {
 			return u, floorSize