feat: embed AppImage update information for delta updates

Add a top-level publish config in electron-builder.json5 pointing to
GitHub Releases. This embeds the update information URL in the AppImage
header, enabling tools like AppImageUpdate, AppImageLauncher, and
AppManager to perform delta updates instead of full re-downloads.

Also update the Linux build workflow to upload the generated .zsync file
alongside the .AppImage artifact.

Fixes #219

electron-builder.json5

@@ -20,14 +20,15 @@
     "!CONTRIBUTING.md",
     "!LICENSE"
   ],
-  "extraResources": [
-    {
-      "from": "public/wallpapers",
-      "to": "assets/wallpapers"
-    }
-  ],
-
-  "mac": {
+  "extraResources": [
+    {
+      "from": "public/wallpapers",
+      "to": "assets/wallpapers"
+    }
+  ],
+  "publish": [{"provider": "github"}],
+
+  "mac": {
     "hardenedRuntime": false,
     "target": [
       {