v0.12.5: drive-type-aware cross-drive backup validation

Fix cross-drive backup failing for system-drive destinations. The
/mnt/hdd_placeholder folder is on the internal SSD, so IsMountPoint()
returned false and the old code hard-blocked it.

- ValidateDestination() (crossdrive.go): onSystemDrive flag replaces
  boolean-only check; system drives require ≥10 GB free and <90% usage
  to protect OS stability; external drives just need ≥100 MB free.
- CheckBackupDestination() (mounts_linux.go): Tier 4 now branches on
  h.SystemDrive; system drive blocks at <10 GB free or ≥90% usage
  (matches runner enforcement); external drive unchanged (warn 90%,
  block 95%). Removes the && h.Severity == "ok" guard that was
  preventing system-drive critical messages from overriding warnings.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

controller/internal/backup/crossdrive.go

@@ -159,9 +159,9 @@ func (r *CrossDriveRunner) IsRunning(stackName string) bool {
 }
 
 // ValidateDestination checks that the destination path exists, is writable,
-// and has sufficient free space (at least 100MB).
-// A non-mount-point destination (e.g. a folder on the system drive) is allowed
-// with a logged warning — consistent with the web UI's CheckBackupDestination.
+// and has sufficient free space. System-drive destinations get stricter limits
+// (≥10 GB free, <90% used) to protect OS stability; external drives just need
+// ≥100 MB. Non-mount-point destinations are allowed with a logged warning.
 func (r *CrossDriveRunner) ValidateDestination(path string) error {
 	if path == "" {
 		return fmt.Errorf("destination path is empty")
@@ -169,15 +169,28 @@ func (r *CrossDriveRunner) ValidateDestination(path string) error {
 	if _, err := os.Stat(path); os.IsNotExist(err) {
 		return fmt.Errorf("destination %s does not exist", path)
 	}
-	if !system.IsMountPoint(path) {
+	onSystemDrive := !system.IsMountPoint(path)
+	if onSystemDrive {
 		r.logger.Printf("[WARN] Destination %s is not a separate mount point (system drive) — backup will proceed but data is not protected against drive failure", path)
 	}
 	if !system.IsWritable(path) {
 		return fmt.Errorf("destination %s is not writable", path)
 	}
-	di := system.GetDiskUsage(path)
-	if di != nil && di.AvailGB < 0.1 {
-		return fmt.Errorf("destination %s has insufficient free space (%.1f GB)", path, di.AvailGB)
+	if di := system.GetDiskUsage(path); di != nil {
+		if onSystemDrive {
+			// System drive: protect OS stability — require ≥10 GB free and <90% used
+			if di.AvailGB < 10 {
+				return fmt.Errorf("destination %s is on the system drive with only %.1f GB free — at least 10 GB required to protect OS stability", path, di.AvailGB)
+			}
+			if di.UsedPercent >= 90 {
+				return fmt.Errorf("destination %s is on the system drive at %.0f%% capacity — maximum 90%% allowed", path, di.UsedPercent)
+			}
+		} else {
+			// External drive: just ensure it's not completely full
+			if di.AvailGB < 0.1 {
+				return fmt.Errorf("destination %s has insufficient free space (%.1f GB free)", path, di.AvailGB)
+			}
+		}
 	}
 	return nil
 }

controller/internal/system/mounts_linux.go

@@ -172,13 +172,28 @@ func CheckBackupDestination(path string) DestinationHealth {
 	if di := GetDiskUsage(path); di != nil {
 		h.UsedPercent = di.UsedPercent
 		h.FreeGB = di.AvailGB
-		if di.UsedPercent >= 95 {
-			h.Warning = fmt.Sprintf("A mentési meghajtó megtelt (%.0f%% használt)!", di.UsedPercent)
-			h.Blocked = true
-			h.Severity = "critical"
-		} else if di.UsedPercent >= 90 && h.Severity == "ok" {
-			h.Warning = fmt.Sprintf("A mentési meghajtó majdnem megtelt (%.0f%% használt).", di.UsedPercent)
-			h.Severity = "warning"
+		if h.SystemDrive {
+			// System drive: stricter limits to protect OS stability
+			if di.AvailGB < 10 {
+				h.Warning = fmt.Sprintf("A rendszermeghajtón csak %.1f GB szabad — legalább 10 GB szükséges a rendszer stabilitásához!", di.AvailGB)
+				h.Blocked = true
+				h.Severity = "critical"
+			} else if di.UsedPercent >= 90 {
+				h.Warning = fmt.Sprintf("A rendszermeghajtó %.0f%%-ban megtelt — maximum 90%% megengedett.", di.UsedPercent)
+				h.Blocked = true
+				h.Severity = "critical"
+			}
+			// If neither triggers, keep the Tier 3 system-drive warning
+		} else {
+			// External drive: original thresholds
+			if di.UsedPercent >= 95 {
+				h.Warning = fmt.Sprintf("A mentési meghajtó megtelt (%.0f%% használt)!", di.UsedPercent)
+				h.Blocked = true
+				h.Severity = "critical"
+			} else if di.UsedPercent >= 90 {
+				h.Warning = fmt.Sprintf("A mentési meghajtó majdnem megtelt (%.0f%% használt).", di.UsedPercent)
+				h.Severity = "warning"
+			}
 		}
 	}