Sanitize file names taken from server metadata (closes #9)

A file title or album name decrypted from server data could name a path
outside the chosen directory (`../../.ssh/authorized_keys`). One module,
src/filename.ts, now makes such names safe for `quak get`/`get-thumb`
without `--out`, downloadFile/downloadThumbnail without outPath, and the
backup and metadata backup trees. Originals-cache extensions are limited to
letters and digits. A user-supplied path is still used as is. decryptFile
reads a missing or non-string title as "" and rejects metadata that is not
a JSON object.

Model: opus-5-5

src/filename.ts

@@ -0,0 +1,37 @@
+// File names built from server-supplied metadata.
+//
+// A file's title and a collection's name are decrypted from data the server
+// hands us, and quak does not trust the server. Any name taken from them and
+// used on disk goes through here, so it can only ever name one file inside the
+// directory the caller chose: never a path, never `..`, never hidden, never a
+// Windows device name.
+//
+// A path the user typed (`--out`, `outPath`) is not passed through here: the
+// caller is trusted, the server is not.
+
+import { extname } from "node:path";
+
+// Path separators, characters Windows forbids in file names, and control
+// characters (NUL included).
+// eslint-disable-next-line no-control-regex
+const UNSAFE_CHARACTERS = /[/\\:*?"<>|\x00-\x1f\x7f]/g;
+
+// Names Windows reserves for devices, with or without an extension.
+const RESERVED_DEVICE_NAME = /^(con|prn|aux|nul|com[1-9]|lpt[1-9])(\.|$)/i;
+
+// `name` made safe to use as a single file name. Each unsafe character becomes
+// `_`, a leading run of dots becomes one `_`, and a device name gets a leading
+// `_`. A name with none of these comes back unchanged. An empty name becomes
+// `fallback`, which the caller derives from the record's ID.
+export const sanitizeFileName = (name: string, fallback: string): string => {
+    if (name === "") return fallback;
+    const cleaned = name.replace(UNSAFE_CHARACTERS, "_").replace(/^\.+/, "_");
+    return RESERVED_DEVICE_NAME.test(cleaned) ? `_${cleaned}` : cleaned;
+};
+
+// The extension of `title` (".jpg"), or ".bin" when it has none or it holds
+// anything but letters and digits.
+export const safeExtension = (title: string): string => {
+    const ext = extname(title);
+    return /^\.[A-Za-z0-9]+$/.test(ext) ? ext : ".bin";
+};