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/model/decrypt.ts

@@ -98,9 +98,18 @@ export const decryptFile = (
         key,
     );
     const metadataJSON = JSON.parse(new TextDecoder().decode(metadataBytes));
+    if (
+        typeof metadataJSON !== "object" ||
+        metadataJSON === null ||
+        Array.isArray(metadataJSON)
+    ) {
+        throw new Error(`file ${raw.id}: metadata is not a JSON object`);
+    }
 
     const metadata: FileMetadata = {
-        title: metadataJSON.title ?? "",
+        // The server controls this JSON: a title that is missing or not a
+        // string becomes "", never an arbitrary value.
+        title: typeof metadataJSON.title === "string" ? metadataJSON.title : "",
         fileType: parseFileType(metadataJSON.fileType ?? -1),
         creationTime: metadataJSON.creationTime ?? 0,
         modificationTime: metadataJSON.modificationTime ?? 0,