feat(image): implement sharp-based image operations backend

Add src/main/ImageOperations.js (convert/resize/compress/rotate via
sharp), modeled on PDFOperations.js's executeOperation dispatcher.
Wire ipcMain.handle('process-image-operation', ...) in main.js using
sanitizeErrorMessage() on error paths, and replace the 5 stale/unused
image-* channel names in preload.js's ALLOWED_SEND_CHANNELS with
process-image-operation + select-image-folder (mirroring
select-pdf-folder for a later batch-UI task).

Amit Haridas

src/preload.js

@@ -45,11 +45,8 @@ const ALLOWED_SEND_CHANNELS = [
   'universal-convert-batch',
 
   // Image converter
-  'image-convert',
-  'image-batch-convert',
-  'image-resize',
-  'image-compress',
-  'image-rotate',
+  'process-image-operation',
+  'select-image-folder',
 
   // Audio converter
   'audio-convert',
@@ -205,6 +202,7 @@ const ALLOWED_RECEIVE_CHANNELS = [
   // Folder selection
   'folder-selected',
   'pdf-folder-selected',
+  'image-folder-selected',
 
   // Header/Footer
   'header-footer-settings-data',