Write each backed-up original once and skip the precache (closes #106)

An original fetched for a backup is now written by the download writer
straight into the backup's originals/, and the content cache records it
there instead of keeping its own copy. One the cache already held is
still copied. `quak backup` opens its library with the thumbnail and
originals precache off, as the one-shot commands do.

Model: opus-5-5

src/library/index.ts

@@ -571,7 +571,8 @@ export class Library {
                 refresh: () => this.refreshNow(),
                 listCollections: () => this.store.listCollections(),
                 listFiles: (id) => this.store.listFiles(id),
-                original: (fileID) => cache!.original(fileID),
+                original: (fileID, destination) =>
+                    cache!.backupOriginal(fileID, destination),
                 thumbnail: (fileID) => cache!.thumbnail(fileID),
             },
             { ...opts, downloadDirectory },