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

test/cli/metadata-backup.test.ts

@@ -165,14 +165,15 @@ const buildMetaMock = async (): Promise<MetaMockState> => {
         },
     };
 
-    // Collection 2: "Work" with no magic metadata
+    // Collection 2: "../Work" with no magic metadata. The server chose a name
+    // that tries to climb out of the backup directory.
     const ck2 = sodium.crypto_secretbox_keygen();
     const { ciphertext: encCK2, nonce: ck2N } = encryptSecretbox(
         ck2,
         masterKey,
     );
     const { ciphertext: encCN2, nonce: cn2N } = encryptSecretbox(
-        new TextEncoder().encode("Work"),
+        new TextEncoder().encode("../Work"),
         ck2,
     );
     const rawColl2 = {
@@ -496,7 +497,8 @@ describe("quak backup-metadata", () => {
         await runBackup(outDir);
 
         const collDirs = readdirSync(join(outDir, "collections"));
-        expect(collDirs.length).toBe(2);
+        // "../Work" is sanitized into one directory name.
+        expect(collDirs.sort()).toEqual(["10-Vacation", "20-__Work"]);
 
         // Find the Vacation collection dir (prefixed with ID)
         const vacDir = collDirs.find((d) => d.includes("Vacation"))!;