v0.9.0: Storage paths registry, per-app HDD_PATH resolution, storage management UI

- Fix backup toggles not appearing (read each app's own HDD_PATH from app.yaml)
- Storage paths registry in settings.json with auto-discovery from deployed apps
- Settings page "Adattárolók" section with disk usage, add/remove/default/schedulable
- Deploy page path field as dropdown of registered storage paths
- Health check storage monitoring (mount point, disk usage alerts)
- Mount-point validation utilities (Linux syscall + cross-platform stubs)
- Controller docker-compose mount changed to /mnt:/mnt:rw for multi-storage

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

controller/internal/system/mounts_other.go

@@ -0,0 +1,49 @@
+//go:build !linux
+
+package system
+
+import (
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+// IsMountPoint always returns true on non-Linux (assume OK for dev/testing).
+func IsMountPoint(_ string) bool { return true }
+
+// IsWritable checks if the given path is writable by attempting to create+remove a temp file.
+func IsWritable(path string) bool {
+	testFile := filepath.Join(path, ".felhom-write-test")
+	f, err := os.Create(testFile)
+	if err != nil {
+		return false
+	}
+	f.Close()
+	os.Remove(testFile)
+	return true
+}
+
+// PathsOverlap returns true if one path is a parent or child of the other.
+func PathsOverlap(a, b string) bool {
+	a = filepath.Clean(a)
+	b = filepath.Clean(b)
+	if a == b {
+		return true
+	}
+	aSep := a + string(os.PathSeparator)
+	bSep := b + string(os.PathSeparator)
+	return strings.HasPrefix(aSep, bSep) || strings.HasPrefix(bSep, aSep)
+}
+
+// DiskUsageInfo holds disk usage statistics for a path.
+type DiskUsageInfo struct {
+	TotalGB     float64
+	UsedGB      float64
+	AvailGB     float64
+	UsedPercent float64
+	TotalHuman  string
+	UsedHuman   string
+}
+
+// GetDiskUsage returns nil on non-Linux.
+func GetDiskUsage(_ string) *DiskUsageInfo { return nil }