feat: letterbox-aware clamp, full-frame image, fit/fill zoom fix

- image_canvas.dart:
  - Fill mod: viewport tabanli clamp (_tx=0'da kilit)
  - Fit mod: RenderBox ile resmin gercek render boyutuna gore clamp
  - Letterbox offset (offX/offY) hesaba katilarak resmin kenarlarina kadar pan
  - Zoom-to-cursor dogru calisiyor (alignment: topLeft ile)
  - Min scale 1.0 (her iki modda da)
  - Overflow durumunda lo/hi reversed clamp (goruntu disari kacmaz)
- viewer_screen.dart: image alani top:24 -> top:0 (tam cerceve)
- install.sh: release -> debug bundle path (gelistirme)

lib/screens/viewer_screen.dart

@@ -119,28 +119,36 @@ class _ViewerScreenState extends State<ViewerScreen> with WindowListener {
             child: Stack(
               children: [
                 Positioned(
-                  top: 24,
+                  top: 0,
                   left: 0,
                   right: 0,
                   bottom: 0,
                   child: Container(color: const Color(0xFF1a1a1a)),
                 ),
-                Container(
-                  color: Colors.transparent,
-                  child: _hasImages
-                      ? ImageCanvas(filePath: ImageManager.instance.currentImagePath)
-                      : const Center(
-                          child: Column(
-                            mainAxisSize: MainAxisSize.min,
-                            children: [
-                              Icon(Icons.image, size: 64, color: Colors.white24),
-                              SizedBox(height: 16),
-                              Text('Resimleri buraya sürükleyin\nveya Ctrl+O ile açın',
-                                  textAlign: TextAlign.center,
-                                  style: TextStyle(color: Colors.white38, fontSize: 16)),
-                            ],
+                Positioned(
+                  top: 0,
+                  left: 0,
+                  right: 0,
+                  bottom: 0,
+                  child: Container(
+                    color: Colors.transparent,
+                    child: _hasImages
+                        ? ImageCanvas(
+                            filePath: ImageManager.instance.currentImagePath,
+                          )
+                        : const Center(
+                            child: Column(
+                              mainAxisSize: MainAxisSize.min,
+                              children: [
+                                Icon(Icons.image, size: 64, color: Colors.white24),
+                                SizedBox(height: 16),
+                                Text('Resimleri buraya sürükleyin\nveya Ctrl+O ile açın',
+                                    textAlign: TextAlign.center,
+                                    style: TextStyle(color: Colors.white38, fontSize: 16)),
+                              ],
+                            ),
                           ),
-                        ),
+                  ),
                 ),
                 const CustomTitleBar(),
                 if (_isDragging)