Check downloaded originals against their recorded content hash (closes #68)

downloadFile, shared by quak get, the content cache and backup, hashes
the decrypted bytes (unkeyed BLAKE2b-512, standard base64) and stores
nothing on a mismatch, failing with an error naming the file ID. A live
photo ZIP is unpacked as it streams with fflate's Unzip, in small
slices so memory stays bounded however far an entry expands, and its
image and video hashed separately as <imageHash>:<videoHash>.
decryptFile reads the older imageHash/videoHash fields for live
photos. A file with no recorded hash is stored unchecked.

Model: opus-5-5

test/crypto/hash.test.ts

@@ -0,0 +1,33 @@
+import { beforeAll, describe, expect, it } from "vitest";
+import {
+    chunkHashFinal,
+    chunkHashInit,
+    chunkHashUpdate,
+    init,
+} from "../../src/crypto/index.js";
+
+beforeAll(async () => {
+    await init();
+});
+
+describe("content hash", () => {
+    // RFC 7693 Appendix A: BLAKE2b-512 of "abc".
+    const abc = Buffer.from(
+        "ba80a53f981c4d0d6a2797b69f12f6e94c212f14685ac4b74b12bb6fdbffa2d1" +
+            "7d87c5392aab792dc252d5de4533cc9518d38aa8dbf1925ab92386edd4009923",
+        "hex",
+    ).toString("base64");
+
+    it("is unkeyed BLAKE2b-512 in standard base64", () => {
+        const state = chunkHashInit();
+        chunkHashUpdate(state, new TextEncoder().encode("abc"));
+        expect(chunkHashFinal(state)).toBe(abc);
+    });
+
+    it("gives the same hash when the input arrives in chunks", () => {
+        const state = chunkHashInit();
+        chunkHashUpdate(state, new TextEncoder().encode("a"));
+        chunkHashUpdate(state, new TextEncoder().encode("bc"));
+        expect(chunkHashFinal(state)).toBe(abc);
+    });
+});