Strip fx call-chain noise from startup errors; clarify file:// error

internal/storage/file.go

@@ -23,9 +23,8 @@ type FileStorer struct {
 // Uses the real OS filesystem by default; call SetFilesystem to override for testing.
 func NewFileStorer(basePath string) (*FileStorer, error) {
 	fs := afero.NewOsFs()
-	// Ensure base path exists
 	if err := fs.MkdirAll(basePath, 0755); err != nil {
-		return nil, fmt.Errorf("creating base path: %w", err)
+		return nil, fmt.Errorf("file:// storage: cannot create or access %s: %w (check that the volume is mounted and writable)", basePath, err)
 	}
 	return &FileStorer{
 		fs:       fs,