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/ISSUE_TEMPLATE/bug_report.md

@@ -1,6 +1,6 @@
 ---
 name: Bug Report
-about: Report a bug to help improve Vocalinux
+about: Report a bug to help improve Voculinux
 title: "[BUG] "
 labels: bug
 assignees: ''
@@ -32,7 +32,7 @@ If applicable, attach screenshots.
 
 - **OS / distro**: (e.g. Ubuntu 24.04)
 - **Python version**: (e.g. 3.12; 3.11+ required)
-- **Vocalinux version**: (`vocalinux --version`)
+- **Voculinux version**: (`voculinux --version`)
 - **Install method**: (installer / AppImage / AUR / Flatpak / Snap / source / PyPI)
 - **Display server**: (X11 / Wayland)
 - **Desktop environment**: (e.g. GNOME, KDE Plasma)
@@ -41,7 +41,7 @@ If applicable, attach screenshots.
 
 ## Debug logs
 
-Run `vocalinux --debug` and paste the relevant output:
+Run `voculinux --debug` and paste the relevant output:
 
 ```
 [paste logs here]