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/main.dart

@@ -25,7 +25,11 @@ void main(List<String> args) async {
 
   await windowManager.waitUntilReadyToShow(windowOptions, () async {
     if (saved != null) {
-      await windowManager.setPosition(saved.position);
+      final position = await WindowPersistence.cascadePosition(
+        previousPosition: saved.position,
+        previousSize: saved.size,
+      );
+      await windowManager.setPosition(position);
       await windowManager.setSize(saved.size);
     } else {
       await windowManager.center();
@@ -33,6 +37,11 @@ void main(List<String> args) async {
     await windowManager.setResizable(true);
     await windowManager.show();
     await windowManager.focus();
+
+    // Açılış konumunu hemen kaydet — sıradaki pencere güncel referans okusun
+    final pos = await windowManager.getPosition();
+    final size = await windowManager.getSize();
+    WindowPersistence.saveSync(position: pos, size: size);
   });
 
   // Extract file paths from command-line arguments