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.

.gitignore

@@ -49,7 +49,7 @@ app.*.map.json
 *.deb
 *.tar.gz
 *.exe
-!dist/imajviewer_1.7.0_amd64.deb
+!dist/imajviewer_1.7.1_amd64.deb
 !dist/imajviewer_setup_1.1.0.exe
 deb/usr/lib/imajviewer/
 release/