feat: jfif/jpe/wbmp format destegi, pencere cascade yerlesimi, konum kaydi duzeltmesi

- file_handler/image_manager: jfif, jpe, wbmp uzantilari
- .desktop + install.sh MimeType: image/vnd.wap.wbmp eklendi
- main.dart: yeni pencere oncekinin saginda acilir (screen_retriever ile ekran tespiti)
- viewer_screen: 2sn polling kaldirildi, konum kaydi tek akisa oturtuldu (acilis + move/resize debounce + close senkron)
- dist/rebuild-deb.sh: .desktop kaynaktan kopyalanir (MimeType/StartupWMClass guncel kalir)
- .deb guncellendi, docs: session-2026-08-03 + plan dokumanlari

lib/services/file_handler.dart

@@ -6,7 +6,7 @@ class FileHandler {
   static Future<void> openFileDialog() async {
     final result = await FilePicker.platform.pickFiles(
       type: FileType.custom,
-      allowedExtensions: ['png', 'jpg', 'jpeg', 'webp', 'bmp', 'gif'],
+      allowedExtensions: ['png', 'jpg', 'jpeg', 'jfif', 'jpe', 'webp', 'bmp', 'gif', 'wbmp'],
       allowMultiple: true,
     );