fix(build): emit linux x86_64 tar.gz on every package run

The in-app updater installs the release asset matched by pickAsset
(tar.gz + linux + x86_64/amd64); the config only built AppImage+deb, so
plain 'npm run package' produced no updatable asset and required a second
electron-builder invocation plus a manual rename (as done for 0.1.13).

electron-builder.yml

@@ -21,6 +21,11 @@ linux:
   target:
     - AppImage
     - deb
+    # tar.gz is what the in-app updater installs (pickAsset looks for a
+    # linux x86_64/amd64 tarball); arch must be x64 so the asset name ends
+    # in -linux-x86_64 like every published release.
+    - target: tar.gz
+      arch: x64
 
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