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

README.md

@@ -687,24 +687,26 @@ Without `--out`, `get` writes the file to the working directory under its title,
 and `get-thumb` writes the thumbnail there under `thumb_` followed by the title,
 each with unsafe characters replaced. A name over 255 bytes, which most file
 systems refuse, is cut short between two characters before its extension, so the
-extension is kept whenever it fits.
+extension is kept whenever it fits. An extension too long to leave room for the
+name's first character is cut short too, so the name never starts with a dot.
 
 `get` and `get-thumb` resolve the file by ID directly, so `--collection` is
 accepted for backward compatibility but ignored. For a live photo, `get` writes
 its image and its video, each named after the title with its own extension, as
-Ente's clients name them (`IMG_0001.heic` and `IMG_0001.mov`). With
-`--out PATH`, the image is written to `PATH` and the video beside it, with
-`PATH`'s name and the video's extension; a `PATH` with the video's extension is
-refused. `backup-metadata --exif` (alias `--all`) additionally fetches each
-file's original through the cache and records, from it or a live photo's image,
-its XMP metadata, its EXIF metadata and, for a JPEG, its dimensions. EXIF is
-read with [exifreader](https://github.com/mattiasw/ExifReader) from any image
-format it reads, JPEG, HEIC/HEIF, AVIF, PNG and WebP among them. The record's
-`exif` field is exifreader's EXIF tag output: each tag by name, with its
-`value`, `description` and `computed` value. An EXIF block exifreader finds but
-reads no tag from is recorded, base64, as `exifRaw`, with the reason in
-`exifError`. `collections`, `files`, `backup`, `helper list-missing-thumbnails`
-and `helper fix-missing-thumbnails` take `--json` for machine-readable output.
+Ente's clients name them (`IMG_0001.heic` and `IMG_0001.mov`), and each name is
+cut short as above once its own extension is in it. With `--out PATH`, the image
+is written to `PATH` and the video beside it, with `PATH`'s name and the video's
+extension; a `PATH` with the video's extension is refused.
+`backup-metadata --exif` (alias `--all`) additionally fetches each file's
+original through the cache and records, from it or a live photo's image, its XMP
+metadata, its EXIF metadata and, for a JPEG, its dimensions. EXIF is read with
+[exifreader](https://github.com/mattiasw/ExifReader) from any image format it
+reads, JPEG, HEIC/HEIF, AVIF, PNG and WebP among them. The record's `exif` field
+is exifreader's EXIF tag output: each tag by name, with its `value`,
+`description` and `computed` value. An EXIF block exifreader finds but reads no
+tag from is recorded, base64, as `exifRaw`, with the reason in `exifError`.
+`collections`, `files`, `backup`, `helper list-missing-thumbnails` and
+`helper fix-missing-thumbnails` take `--json` for machine-readable output.
 
 `backup --verify` also hashes the originals already in the backup and replaces
 any that do not match the content hash Ente records; one it cannot replace stays
@@ -864,11 +866,12 @@ gets its ID added as well: beside an album `x`, whose JSON is `x.json`, an album
 named `x.json` gets the directory `x.json (12)/`. A name too long for a file
 name, which most file systems limit to 255 bytes, gets its ID added too. A name
 with its ID added is cut short between two characters in the part before the ID,
-just far enough that it fits. When a symlink's extension alone leaves no room
-for the ID, the extension is cut short and the part before the ID keeps only the
-bytes that cut leaves over. A collection's directory name also leaves room for
-the `.json` its JSON adds. IDs never change, so a name stays the same from run
-to run until such a clash appears or goes away.
+just far enough that it fits. When a symlink's extension leaves no room for the
+ID and the first character of the name, the extension is cut short, and the part
+before the ID keeps that character and any bytes the cut leaves over. A
+collection's directory name also leaves room for the `.json` its JSON adds. IDs
+never change, so a name stays the same from run to run until such a clash
+appears or goes away.
 
 A collection whose directory or JSON cannot be written, such as one whose
 directory name is taken by a file you put in `collections/`, does not stop the