Names quak builds from server data fit in 255 bytes (closes #205)

The default names of `quak get`, `quak get-thumb`, `downloadFile` and `downloadThumbnail`, and the album folders of `backup-metadata`, could be over 255 bytes, which a file system refuses. `defaultFileName` and `shortenKeepingExtension` in `src/filename.ts` now cut such a name short, keeping its extension when it fits; a live photo's image and video get their own extension before the cut. A cut name never starts with a dot and is never a Windows device name. `--out` and `outPath` are kept as given. A `backup-metadata` folder keeps its `<id>-`. `safeExtension` gives `.bin` for an extension of more than 16 letters and digits, so a save path's name always fits.

Model: opus-5-5

README.md

@@ -683,27 +683,41 @@ before any request with one line, `Invalid file ID: <arg>` or
 `Invalid collection ID: <arg>`, and exit status 1. One bad `--file` ID refuses
 the whole command, so nothing is fixed.
 
+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. 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
 as it was and goes into `failures.json` (see "Backup layout").
 
+`backup-metadata <dir>` writes `account.json` and, for each collection, a
+directory `collections/<id>-<name>/` holding `_collection.json` and a
+`<fileID>.json` for each of its files. The collection's name has unsafe
+characters replaced and is cut short between two characters so that the
+directory's name fits in 255 bytes; the ID keeps the directories apart.
+
 `backup-metadata` fetches ML data in requests of up to 200 files. When a request
 fails, the error is logged, each of its files is written with the reason in an
 `mlDataError` field instead of `mlData`, and the dump goes on. A file whose ML
@@ -756,9 +770,10 @@ Each original is saved at its save path, the same path `photo.savePath` gives
 and `photo.download()` writes (see Read surface below). The date is the photo's
 `takenAt` (the date set in Ente if it was edited, else its creation time) in the
 time zone of the machine running quak. The extension is the one in the file's
-name as uploaded, case kept, or `.bin` when it has none or it holds anything but
-letters and digits. When the date or the time zone changes, the next run saves
-the original at its new path and leaves the old copy where it is.
+name as uploaded, case kept, or `.bin` when it has none, it holds anything but
+letters and digits, or it is more than 16 of them long. When the date or the
+time zone changes, the next run saves the original at its new path and leaves
+the old copy where it is.
 
 `account.json` holds the account's `email` and `userID`, as `backup-metadata`
 writes it. A collection's JSON holds its `id`, `name`, `type`, `ownerID`,
@@ -851,11 +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