Fix: restore allowDowngrade=true for nightly channel to enable periodic nightly update checks

The previous change hardcoded allowDowngrade=false, which breaks periodic update
checks after a stable-to-nightly switch. Since nightly pre-release versions
(e.g. 0.0.17-nightly.*) are semver-lower than their stable counterparts
(e.g. 0.0.17), electron-updater needs allowDowngrade=true on the nightly channel
to discover these updates during startup, poll, and menu-triggered checks.

Author: cursoragent

apps/desktop/src/main.ts

@@ -1139,9 +1139,9 @@ function createBaseUpdateState(
 function applyAutoUpdaterChannel(channel: DesktopUpdateChannel): void {
   autoUpdater.channel = channel;
   autoUpdater.allowPrerelease = channel === "nightly";
-  autoUpdater.allowDowngrade = false;
+  autoUpdater.allowDowngrade = channel === "nightly";
   console.info(
-    `[desktop-updater] Using update channel '${channel}' (allowPrerelease=${channel === "nightly"}, allowDowngrade=false).`,
+    `[desktop-updater] Using update channel '${channel}' (allowPrerelease=${channel === "nightly"}, allowDowngrade=${channel === "nightly"}).`,
   );
 }