Add font import for Now, Gotham and Open Sauce One overrides; package fonts for Void

Font import:
- Migration M6 adds user_fonts; files are stored content-addressed under
  the local data dir. New Rust commands inspect, import, list, read and
  remove fonts, refusing WOFF2, collections, variable fonts and
  restricted fsType, and unpacking WOFF1 to plain sfnt.
- Imported fonts register in the render worker as user:<Face> families
  with an italic alias, and are pinned in the frozen render prefs so
  issued invoices can still re-render. Fonts referenced by issued
  invoices are hidden, never deleted.
- Size and tracking compensations tuned for the substitutes apply only
  while the face is not overridden; default output and fingerprint
  goldens are unchanged.
- Settings gets a Fonts section with a licence acknowledgement.

Void packaging:
- Install fonts to /usr/share/fonts/voiced with per-family OFL licences,
  set font_dirs, and add INSTALL/REMOVE scripts that run fc-cache.
  xbps-create carries both scripts (verified in a void-glibc container).
- Fix xbps-create dependency specs in build-xbps.sh.

src-tauri/src/lib.rs

@@ -141,6 +141,11 @@ pub fn run() {
             commands::archive::archive_pdf,
             commands::archive::read_archive,
             commands::archive::archive_status,
+            commands::fonts::inspect_font,
+            commands::fonts::import_font,
+            commands::fonts::list_user_fonts,
+            commands::fonts::get_user_font_bytes,
+            commands::fonts::remove_user_font,
             commands::files::write_export_file,
             commands::files::get_last_export_dir,
             commands::files::reveal_in_folder,