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

.github/workflows/distro-test-matrix.yml

@@ -11,7 +11,7 @@ on:
       - 'uv.lock'
       - 'requirements/**'
       - 'src/vocalinux/version.py'
-      - 'vocalinux.desktop'
+      - 'voculinux.desktop'
       - '.github/workflows/distro-test-matrix.yml'
   pull_request:
     branches: [ main, master ]
@@ -23,7 +23,7 @@ on:
       - 'uv.lock'
       - 'requirements/**'
       - 'src/vocalinux/version.py'
-      - 'vocalinux.desktop'
+      - 'voculinux.desktop'
       - '.github/workflows/distro-test-matrix.yml'
   workflow_dispatch: