Linux: accept file:// URI args so desktop launchers recognise pdfcraft as PDF default (#382)

* Linux: accept file:// URI args so desktop launchers recognise pdfcraft as PDF default

GNOME Files (and other freedesktop-spec launchers) only register an app as the
default handler for a mimetype if its .desktop Exec accepts URIs (%u/%U), not
just paths (%F). pdfcraft's entry used %F, so even after `xdg-mime default
ai.storyteller.pdfcraft.desktop application/pdf`, double-clicking a PDF in
Files silently fell back to another installed viewer.

Switch Exec to %u and decode file:// args back to plain paths in main.rs
before they reach the rest of the app, which only ever opens paths. Other
schemes and plain CLI paths pass through unchanged.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

* path_from_arg: file://localhost, foreign hosts left alone, Windows drive paths, no indexing; Exec %U (several files)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Dmitriy Bunakalya <dmitriy.bunakalya@appfollow.io>
Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>
Co-authored-by: Brandon Thomas <echelon@gmail.com>

packaging/linux/ai.storyteller.pdfcraft.desktop

@@ -3,12 +3,12 @@ Type=Application
 Name=PdfCraft
 GenericName=PDF Editor
 Comment=Read, organize, combine, split and secure PDFs
-Exec=pdfcraft %F
+Exec=pdfcraft %U
 TryExec=pdfcraft
 Icon=ai.storyteller.pdfcraft
 Terminal=false
 StartupNotify=true
 StartupWMClass=pdfcraft
-Categories=Office;Viewer;Graphics;
+Categories=Office;Viewer;
 Keywords=pdf;viewer;editor;annotate;sign;forms;merge;split;
 MimeType=application/pdf;