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

TODO.md

@@ -17,9 +17,11 @@ pre-1.0
 
 # Next Step
 
-No bug is open. The check of quak against a live Ente account,
-https://git.eeqj.de/sneak/quak/issues/187, needs an account only sneak has, so
-it waits on him.
+No bug is open. Two changes sneak asked for are open:
+https://git.eeqj.de/sneak/quak/issues/211 and
+https://git.eeqj.de/sneak/quak/issues/212. The check of quak against a live Ente
+account, https://git.eeqj.de/sneak/quak/issues/187, needs an account only sneak
+has, so it waits on him.
 
 Tagging and releases are decided by sneak alone, and happen only when he
 declares one.
@@ -27,13 +29,17 @@ declares one.
 # Completed Steps
 
 - 2026-10-18: Names quak builds from an album name or a title fit in a file name
-  (issue 205). `shortenKeepingExtension` in `src/filename.ts` cuts a name short
-  to a number of bytes, keeping its extension when it fits, and can put text
-  right before the extension; the backup's `withID` uses it for an ID. The
-  default names of `quak get`, `quak get-thumb`, `downloadFile` and
-  `downloadThumbnail` are cut short with it to 255 bytes, `thumb_` included.
-  `backup-metadata` cuts an album's name short so that its folder,
-  `<id>-<name>`, fits in 255 bytes. `safeExtension` gives `.bin` for an
+  (https://git.eeqj.de/sneak/quak/issues/205). `shortenKeepingExtension` in
+  `src/filename.ts` cuts a name short to a number of bytes, keeping its
+  extension when it fits and always its first character, and can put text right
+  before the extension; the backup's `withID` uses it for an ID.
+  `defaultFileName` there gives the name a file is saved under when the caller
+  gives no path: a prefix such as `thumb_` and the title with unsafe characters
+  replaced, cut short with it to 255 bytes, and, for a live photo's image or
+  video, with that part's own extension before the cut. `quak get`,
+  `quak get-thumb`, `downloadFile` and `downloadThumbnail` use it without
+  `--out` or `outPath`. `backup-metadata` cuts an album's name short so that its
+  folder, `<id>-<name>`, fits in 255 bytes. `safeExtension` gives `.bin` for an
   extension of more than 16 letters and digits, so a save path's name always
   fits.