feat(security): verify binaries with pinned Ed25519 signature on SHA256SUMS

Replace the same-origin checksum with a detached Ed25519 signature
(SHA256SUMS.sig) verified against a pinned public key before extraction,
closing #308: a compromised download mirror can no longer certify a
tampered binary. The signed manifest also binds the release version,
rejecting a forced downgrade to an older signed build.

Verification is mandatory and non-bypassable on the official download path;
custom CLOAKBROWSER_DOWNLOAD_URL mirrors keep the legacy skippable checksum.
Silent auto-update is preserved for everyone because only a constant public
key is pinned, not per-version hashes. Older installed wrappers are
unaffected — the version= line is ignored by their checksum parser.

Python uses cryptography; JS uses node:crypto. Adds tamper, downgrade, and
fail-closed tests in both languages.

js/src/config.ts

@@ -37,6 +37,19 @@ export const PLATFORM_CHROMIUM_VERSIONS: Record<string, string> = {
   "windows-x64": "146.0.7680.177.5",
 };
 
+// ---------------------------------------------------------------------------
+// Ed25519 public keys for verifying downloaded binaries.
+//
+// Each release publishes SHA256SUMS and a detached signature SHA256SUMS.sig.
+// The wrapper verifies that signature against the keys below before trusting
+// any hash in the manifest, so the download origin alone cannot certify a
+// tampered binary. Values are base64 of the 32-byte raw public key. Multiple
+// entries are accepted to allow key rotation. Keep in parity with config.py.
+// ---------------------------------------------------------------------------
+export const BINARY_SIGNING_PUBKEYS: string[] = [
+  "MKFKwIhUcKWq5xTuNA0Ovg99njcDEcEJvmWYYhApvaU=",
+];
+
 // ---------------------------------------------------------------------------
 // Platform detection
 // ---------------------------------------------------------------------------