feat(identity): rename user-visible identity to voculinux with one-time legacy import

The fork takes its own name everywhere a user can see it: binaries
(voculinux, voculinux-gui), desktop entry, autostart, tray, D-Bus
namespace (com.voculinux.Voculinux, no alias for the old name),
config/data/cache dirs, single-instance lock, IBus engine identity,
tray icons, resource prefixes, VOCULINUX_* environment names, and the
update-checker user agent. The internal Python module stays vocalinux
and the distribution metadata keeps the upstream URLs as provenance.

On first run a previous Vocalinux install's config.json, custom
dictionary, and portal restore token are copied once into the new
locations (new config always wins, no merge, no write-back); models
re-resolve through the pinned-checksum download path. An offline
naming-sweep test proves no old-name strings remain on user-visible
surfaces outside the allowlist.

Refs #18

install.d/system_dependencies.sh

@@ -565,7 +565,7 @@ install_system_dependencies() {
             print_error "Unsupported distribution family: $DISTRO_FAMILY"
             print_info ""
             print_info "Your distribution ($DISTRO_NAME) is not officially supported."
-            print_info "However, you can still install Vocalinux manually:"
+            print_info "However, you can still install Voculinux manually:"
             print_info ""
             print_info "1. Run the dependency checker:"
             print_info "   bash scripts/check-system-deps.sh"