GUI: harden cross-platform updater integrity and rollback (#389)

* GUI: verify updater releases by commit and SHA-256

* GUI: add updater rollback and version safeguards

src/SharpEmu.GUI/Languages/en.json

@@ -169,5 +169,6 @@
   "Updater.Status.Installing": "Installing update…",
   "Updater.Status.Timeout": "Update check timed out after 10 seconds.",
   "Updater.Status.Failed": "Could not check for updates.",
+  "Updater.Status.ChecksumFailed": "Downloaded update failed SHA-256 verification.",
   "Updater.Status.Unsupported": "Automatic updating requires a Windows, Linux or macOS x64 build."
 }