fix: make atomic saves durable and preserve the file

writeAtomic renamed the temporary file over the target without syncing
either: on ext4 with delayed allocation a power cut just after a save
could leave a zero-length script, the exact failure autosave exists to
prevent. Sync the temp file before the rename and the directory after
it. While here, keep an existing file's permissions instead of forcing
0o644, and resolve symlinks first so saving through a link replaces
the file it points to, not the link itself (Dropbox/dotfile setups).

app.go

@@ -386,20 +386,48 @@ func (a *App) saveDialog(title, current, ext string, filters []runtime.FileFilte
 	return path, nil
 }
 
+// writeAtomic writes data to path atomically and durably: a temporary file
+// in the same directory is written, given the file's permissions and synced
+// to disk, then renamed over path and the directory itself synced, so a
+// power cut just after a save cannot leave an empty script behind (the very
+// failure autosave, which calls this every 1.2 s, exists to prevent). A
+// symlinked path is resolved first: the file it points to is replaced, not
+// the link. An existing file keeps its permissions; a new one gets 0o644.
 func writeAtomic(path string, data []byte) error {
+	if target, err := filepath.EvalSymlinks(path); err == nil {
+		path = target
+	}
+	mode := os.FileMode(0o644)
+	if st, err := os.Stat(path); err == nil {
+		mode = st.Mode().Perm()
+	}
 	tmp, err := os.CreateTemp(filepath.Dir(path), "."+filepath.Base(path)+".*")
 	if err != nil {
 		return err
 	}
-	_, werr := tmp.Write(data)
-	cerr := tmp.Close()
-	if err := errors.Join(werr, cerr); err != nil {
+	if _, err = tmp.Write(data); err == nil {
+		err = tmp.Chmod(mode)
+	}
+	if err == nil {
+		err = tmp.Sync()
+	}
+	if cerr := tmp.Close(); err == nil {
+		err = cerr
+	}
+	if err != nil {
 		os.Remove(tmp.Name())
 		return err
 	}
-	if err := os.Chmod(tmp.Name(), 0o644); err != nil {
+	if err := os.Rename(tmp.Name(), path); err != nil {
 		os.Remove(tmp.Name())
 		return err
 	}
-	return os.Rename(tmp.Name(), path)
+	// The rename is a directory entry: sync the directory so it too
+	// survives a crash.
+	dir, err := os.Open(filepath.Dir(path))
+	if err != nil {
+		return err
+	}
+	defer dir.Close()
+	return dir.Sync()
 }