fix: keep non-ASCII names readable in zip downloads

The zip writer stores entry names as UTF-8 but did not set the UTF-8 flag
(general purpose bit 11), so readers decoded them as CP437 or Latin-1.
Any model, category or file name with a non-ASCII character came out
garbled, including when the zip was imported back into Thingport.

backend/src/utils/zipWriter.ts

@@ -4,6 +4,11 @@ import zlib from "node:zlib";
 
 // Minimal streaming ZIP writer on node:zlib; yauzl is read-only.
 
+// Bit 3: the CRC and sizes follow the data, in a descriptor. Bit 11: names are UTF-8. Without it readers
+// decode names as CP437 (yauzl, Python) or Latin-1 (fflate), garbling any non-ASCII model, category or
+// file name.
+const ENTRY_FLAGS = 0x0808;
+
 const CRC_TABLE = (() => {
   const table = new Uint32Array(256);
   for (let n = 0; n < 256; n++) {
@@ -70,7 +75,7 @@ export async function writeZip(destPath: string, entries: ZipEntryDescriptor[]):
       const localHeader = Buffer.alloc(30);
       localHeader.writeUInt32LE(0x04034b50, 0);
       localHeader.writeUInt16LE(20, 4); // version needed
-      localHeader.writeUInt16LE(0x0008, 6); // flags: data descriptor follows
+      localHeader.writeUInt16LE(ENTRY_FLAGS, 6); // flags
       localHeader.writeUInt16LE(8, 8); // method: deflate
       localHeader.writeUInt16LE(time, 10);
       localHeader.writeUInt16LE(date, 12);
@@ -125,7 +130,7 @@ export async function writeZip(destPath: string, entries: ZipEntryDescriptor[]):
       central.writeUInt32LE(0x02014b50, 0);
       central.writeUInt16LE(20, 4); // version made by
       central.writeUInt16LE(20, 6); // version needed
-      central.writeUInt16LE(0x0008, 8); // flags
+      central.writeUInt16LE(ENTRY_FLAGS, 8); // flags
       central.writeUInt16LE(8, 10); // method
       central.writeUInt16LE(rec.time, 12);
       central.writeUInt16LE(rec.date, 14);

backend/tests/zipWriter.test.ts

@@ -0,0 +1,35 @@
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import { afterAll, describe, expect, it } from "vitest";
+import { writeZip } from "../src/utils/zipWriter";
+import { listZipEntries, readZipEntry } from "../src/utils/zipReader";
+
+// Zip downloads name entries after models, categories and files, which are often not ASCII. A reader only
+// decodes a name as UTF-8 when the entry is flagged as such, so the round trip goes through the app's own
+// reader, the one behind zip imports and the consume folder.
+
+const dir = fs.mkdtempSync(path.join(os.tmpdir(), "thingport-zip-writer-"));
+
+afterAll(() => fs.rmSync(dir, { recursive: true, force: true }));
+
+describe("writeZip", () => {
+  it("keeps non-ASCII names when the zip is read back", async () => {
+    const source = path.join(dir, "benchy.stl");
+    fs.writeFileSync(source, "solid benchy endsolid");
+    const names = [
+      "Gyvūnai/Šuniukas – Benchy/šuniukas.stl",
+      "Unassigned/哨子/哨子.3mf",
+      "Figurines/Pokémon Pikachu/supporting/notes.txt",
+      "Tools/Cable clip/clip.stl",
+    ];
+    const zipPath = path.join(dir, "names.zip");
+    await writeZip(
+      zipPath,
+      names.map((arcname) => ({ arcname, filePath: source })),
+    );
+
+    expect((await listZipEntries(zipPath)).map((e) => e.name)).toEqual(names);
+    expect((await readZipEntry(zipPath, names[0], 1024))?.toString()).toBe("solid benchy endsolid");
+  });
+});