Fix .deb source staging — preserve app/ subdir for Dockerfile

build-deb.sh used 'cp -r app/ source/' which renames app to source
when source doesn't yet exist, dropping the app/ wrapper that the
Dockerfile's COPY app/ /app/ depends on. The 2>/dev/null || true
on the cp lines hid the resulting failures, so the .deb shipped a
broken /opt/dupfinder/source/ that build-from-source could not use.

Pre-create the source dir and copy each item to its explicit
destination path. Bump package version to 1.0.1.

Also rework dupfinder-setup.sh's image-prep step: prefer a local
image, then a quiet registry pull, then build from the bundled
source. Removes the loud registry-not-found error that scared users
when the (unpublished) tocmo0nlord/dupfinder image wasn't on Docker
Hub.

debian/files/usr/share/applications/dupfinder.desktop

@@ -0,0 +1,12 @@
+[Desktop Entry]
+Type=Application
+Version=1.0
+Name=DupFinder
+GenericName=Duplicate Photo Finder
+Comment=Find and review duplicate photos and videos
+Exec=dupfinder open
+Icon=dupfinder
+Terminal=false
+Categories=Graphics;Photography;Utility;
+Keywords=duplicate;dedup;dedupe;photos;videos;
+StartupNotify=false