feat: comprehensive model management enhancements and platform fixes

- Block 1: Refactored path logic into a centralized PathResolver for OS-agnostic and relative path handling.
- Block 2: Improved platform compatibility (shutdown, macOS menu, System Tray fallback).
- Block 3: Added Power-User features (Model Updater, Smart Cleanup, Thumbnails, Node Sync, Remote Status).
- Fixes: Recursive subfolder scanning, archive detection logic, and UI progress throttling.

src/main/java/de/tki/comfymodels/service/IThumbnailService.java

@@ -0,0 +1,13 @@
+package de.tki.comfymodels.service;
+
+import de.tki.comfymodels.domain.ModelInfo;
+
+/**
+ * Service for downloading or generating model thumbnails.
+ */
+public interface IThumbnailService {
+    /**
+     * Downloads a preview image for the given model and saves it as .preview.png.
+     */
+    void downloadThumbnail(ModelInfo model, java.util.function.Consumer<Boolean> onResult);
+}