A live photo's default names fit, and no cut name starts with a dot

`defaultFileName` in `src/filename.ts` now gives the name a file is saved
under when the caller gives no path, and `originalName`, `thumbnailName`,
`downloadFile` and `downloadThumbnail` all call it. For a live photo's image
and video it puts the part's own extension in before cutting the name short,
so those names fit too; a path from `--out` or `outPath` is kept as given.
`shortenKeepingExtension` keeps the first character before the extension,
cutting a long extension short to leave room for it, so a cut name no longer
starts with a dot. The backup's links for such a title change to match.
TODO.md names the open bug and describes what the code does.

Model: opus-5-5

src/cli-commands.ts

@@ -332,8 +332,8 @@ export const getCommand = async (
         const result = await photo.original();
         // Default name is the file's own title, as the pre-library CLI used
         // (not the editedName-preferring projection title) (issue #52).
-        const outPath = opts.out ?? originalName(file);
         if (result.videoPath === undefined) {
+            const outPath = opts.out ?? originalName(file);
             copyFileSync(result.path, outPath);
             ctx.stderr.write(`${result.bytes} bytes -> ${outPath}\n`);
             return 0;
@@ -341,9 +341,13 @@ export const getCommand = async (
         // A live photo is written as its image and its video, each named after
         // the title with its own extension, as Ente's clients name them. With
         // --out, the image goes there and the video beside it.
-        const imageOut =
-            opts.out ?? withExtension(outPath, extname(result.path));
-        const videoOut = withExtension(outPath, extname(result.videoPath));
+        const imageExt = extname(result.path);
+        const videoExt = extname(result.videoPath);
+        const imageOut = opts.out ?? originalName(file, imageExt);
+        const videoOut =
+            opts.out === undefined
+                ? originalName(file, videoExt)
+                : withExtension(opts.out, videoExt);
         if (imageOut.toLowerCase() === videoOut.toLowerCase()) {
             ctx.stderr.write(
                 `File ${fileID} is a live photo, and its video would also be written to ${imageOut}\n`,