fix: Windows JSON patch via node, Linux AppImage via APPIMAGE_EXTRACT_AND_RUN

Windows: ConvertTo-Json outputs True/False (invalid JSON) and UTF-16
BOM, corrupting tauri.conf.json. Switch to `node -e` which round-trips
JSON correctly.

Linux: linuxdeploy is itself an AppImage and cannot execute inside
Docker without FUSE. APPIMAGE_EXTRACT_AND_RUN=1 makes it self-extract
and run as a plain binary instead.

Also fix unreachable_code warning in check_for_update.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -61,14 +61,13 @@ async fn check_for_update(app: tauri::AppHandle) -> Result<UpdateInfo, String> {
     #[cfg(not(any(target_os = "android", target_os = "ios")))]
     {
         use tauri_plugin_updater::UpdaterExt;
-        match app.updater().map_err(|e| e.to_string())?.check().await {
-            Ok(Some(update)) => {
-                return Ok(UpdateInfo { available: true, version: Some(update.version) });
-            }
-            Ok(None) => return Ok(UpdateInfo { available: false, version: None }),
-            Err(e) => return Err(e.to_string()),
-        }
+        return match app.updater().map_err(|e| e.to_string())?.check().await {
+            Ok(Some(update)) => Ok(UpdateInfo { available: true, version: Some(update.version) }),
+            Ok(None) => Ok(UpdateInfo { available: false, version: None }),
+            Err(e) => Err(e.to_string()),
+        };
     }
+    #[cfg(any(target_os = "android", target_os = "ios"))]
     Ok(UpdateInfo { available: false, version: None })
 }