feat: zoom percentage badge at bottom center (v1.7.1)

- Percentage shown between the prev/next buttons, together with the hover
  controls, on a translucent background.
- Value is the on-screen scale relative to the image's real pixel size
  (100% = 1:1); updated on wheel zoom, fill/fit toggle, image change and
  window resize.
- Pixel-size header parsing moved to lib/services/image_header.dart, shared
  with the settings file-properties panel.
- dist: 1.7.1 .deb replaces the tracked 1.7.0 package.

lib/services/image_header.dart

@@ -0,0 +1,80 @@
+import 'dart:io';
+import 'dart:typed_data';
+
+import 'package:flutter/widgets.dart' show Size;
+import 'package:image/image.dart' as img;
+
+/// Görüntü dosyasının gerçek piksel boyutu — tam decode yapmadan.
+///
+/// PNG/GIF/BMP/WebP: boyut ilk 33 baytta → elle parse.
+/// JPEG: SOF büyük APP/EXIF segmentlerinin ardından gelebilir → package:image startDecode.
+Size? pixelSizeOfFile(String path) {
+  try {
+    final file = File(path);
+    final stat = file.statSync();
+    const maxHead = 512 * 1024;
+    final raf = file.openSync();
+    final header = raf.readSync(stat.size < maxHead ? stat.size : maxHead);
+    raf.closeSync();
+    return _headerPixelSize(header);
+  } catch (_) {}
+  return null;
+}
+
+Size? _headerPixelSize(Uint8List b) {
+  try {
+    if (b.length >= 24 && b[0] == 0x89 && b[1] == 0x50) {
+      // PNG
+      final w = (b[16] << 24) | (b[17] << 16) | (b[18] << 8) | b[19];
+      final h = (b[20] << 24) | (b[21] << 16) | (b[22] << 8) | b[23];
+      return Size(w.toDouble(), h.toDouble());
+    }
+    if (b.length >= 10 && b[0] == 0x47 && b[1] == 0x49 && b[2] == 0x46) {
+      // GIF (WBMP da GIF tabanlı olabilir)
+      final w = b[6] | (b[7] << 8);
+      final h = b[8] | (b[9] << 8);
+      return Size(w.toDouble(), h.toDouble());
+    }
+    if (b.length >= 26 && b[0] == 0x42 && b[1] == 0x4D) {
+      // BMP
+      final w = b[18] | (b[19] << 8) | (b[20] << 16) | (b[21] << 24);
+      final h = b[22] | (b[23] << 8) | (b[24] << 16) | (b[25] << 24);
+      return Size(w.toDouble(), h.abs().toDouble());
+    }
+    if (b.length >= 30 &&
+        b[0] == 0x52 &&
+        b[1] == 0x49 &&
+        b[2] == 0x46 &&
+        b[3] == 0x46 &&
+        b[8] == 0x57 &&
+        b[9] == 0x45 &&
+        b[10] == 0x42 &&
+        b[11] == 0x50) {
+      // WebP
+      if (b[12] == 0x56 && b[13] == 0x50 && b[15] == 0x58) {
+        // VP8X
+        final w = b[24] | (b[25] << 8) | (b[26] << 16);
+        final h = b[27] | (b[28] << 8) | (b[29] << 16);
+        return Size((w + 1).toDouble(), (h + 1).toDouble());
+      }
+      if (b.length >= 25 && b[12] == 0x56 && b[13] == 0x50 && b[15] == 0x4C && b[20] == 0x2F) {
+        // VP8L
+        final w = (b[21] & 0x3F) | ((b[22] & 0x0F) << 6);
+        final h = (b[23] & 0x3F) | ((b[24] & 0x0F) << 6);
+        return Size((w + 1).toDouble(), (h + 1).toDouble());
+      }
+      if (b[12] == 0x56 && b[13] == 0x50 && b[15] == 0x20) {
+        // VP8 (lossy)
+        final w = (b[26] | (b[27] << 8)) & 0x3FFF;
+        final h = (b[28] | (b[29] << 8)) & 0x3FFF;
+        return Size(w.toDouble(), h.toDouble());
+      }
+    }
+  } catch (_) {}
+  try {
+    // JPEG
+    final info = img.JpegDecoder().startDecode(b);
+    if (info != null) return Size(info.width.toDouble(), info.height.toDouble());
+  } catch (_) {}
+  return null;
+}