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

pubspec.yaml

@@ -18,6 +18,7 @@ dependencies:
   flutter:
     sdk: flutter
   window_manager: ^0.4.3
+  screen_retriever: ^0.2.2
   file_picker: ^8.0.0
   path: ^1.9.0
   desktop_drop: ^0.7.1