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/output.test.ts

@@ -64,6 +64,24 @@ describe("CLI file output (issue #52)", () => {
         expect(thumbnailName(renamedFile)).toBe(`thumb_${RAW_TITLE}`);
     });
 
+    it("sanitizes the title when naming `quak get` downloads", () => {
+        // Without `--out`, the server-supplied title names the file, so it must
+        // not be able to point outside the working directory.
+        const hostile = {
+            ...renamedFile,
+            metadata: { ...renamedFile.metadata, title: "../../.bashrc" },
+        };
+        expect(originalName(hostile)).toBe("__.._.bashrc");
+        expect(thumbnailName(hostile)).toBe("thumb___.._.bashrc");
+
+        const untitled = {
+            ...renamedFile,
+            metadata: { ...renamedFile.metadata, title: "" },
+        };
+        expect(originalName(untitled)).toBe("file-100");
+        expect(thumbnailName(untitled)).toBe("thumb_file-100");
+    });
+
     it("does not use the editedName/editedTime projection", () => {
         const record = deriveRecords([], [renamedFile]).photos.get(100);
         // The projection prefers the edits and reports milliseconds; the CLI