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

snap/gui/voculinux.desktop

@@ -1,15 +1,15 @@
 [Desktop Entry]
 Version=1.0
 Type=Application
-Name=Vocalinux
+Name=Voculinux
 Comment=Free, offline voice dictation for Linux
 GenericName=Voice Typing
-Exec=vocalinux
-Icon=${SNAP}/usr/share/icons/hicolor/scalable/apps/vocalinux.svg
+Exec=voculinux
+Icon=${SNAP}/usr/share/icons/hicolor/scalable/apps/voculinux.svg
 Terminal=false
 Categories=Utility;Accessibility;AudioVideo;
 Keywords=voice;typing;dictation;speech;recognition;transcription;accessibility;
 StartupNotify=true
-StartupWMClass=com.vocalinux.Vocalinux
+StartupWMClass=com.voculinux.Voculinux
 X-GNOME-UsesNotifications=true
-X-SnapInstanceName=vocalinux
+X-SnapInstanceName=voculinux