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/interactive.sh

@@ -27,7 +27,7 @@ run_interactive_install() {
 
 EOF
 
-    echo "Welcome! This guided installation will help you set up Vocalinux"
+    echo "Welcome! This guided installation will help you set up Voculinux"
     echo "with the best options for your system."
     echo ""
     echo "Local engines (whisper.cpp, Whisper, Faster Whisper, VOSK) process audio on-device after you download a model."
@@ -38,11 +38,11 @@ EOF
     # and update in place instead of redoing first-time setup. Choosing the full
     # guided setup continues below and can re-point the engine deliberately.
     if [[ "${EXISTING_INSTALL:-no}" == "yes" ]]; then
-        echo "An existing Vocalinux installation was detected:"
+        echo "An existing Voculinux installation was detected:"
         echo "  Configuration: $CONFIG_DIR/config.json (kept unchanged by an upgrade)"
         echo "  Virtual env:   $VENV_DIR (reused by an upgrade)"
         echo ""
-        echo "  1. Upgrade (keep current settings) - update Vocalinux only"
+        echo "  1. Upgrade (keep current settings) - update Voculinux only"
         echo "  2. Full guided setup - choose engine, backend, and model options"
         echo ""
 
@@ -330,7 +330,7 @@ EOF
         echo "  Remote Server: $REMOTE_DISPLAY"
     fi
     echo "  Models: $MODELS_DISPLAY"
-    echo "  Install Location: ${INSTALL_DIR:-\$HOME/.local/share/vocalinux}"
+    echo "  Install Location: ${INSTALL_DIR:-\$HOME/.local/share/voculinux}"
     echo ""
     read_prompt -p "Press Enter to continue with installation, or Ctrl+C to cancel..."
     echo ""