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.

pubspec.yaml

@@ -1,7 +1,7 @@
 name: imajviewer
 description: "Ultra-lightweight image viewer for Linux and Windows."
 publish_to: 'none'
-version: 1.7.0+16
+version: 1.7.1+17
 
 msix_config:
   display_name: ImajViewer