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

packaging/nix/pywhispercpp.nix

@@ -52,7 +52,7 @@ buildPythonPackage rec {
   env = {
     # The sdist ships no version.txt; setup.py falls back to this env var.
     PYWHISPERCPP_VERSION = version;
-    # Build Vulkan as a shared backend: Vocalinux's GPU detector looks for
+    # Build Vulkan as a shared backend: Voculinux's GPU detector looks for
     # libggml-vulkan.so. CMake's build-tree RPATH can leak /build into the
     # wheel; disable CMake RPATHs and let autoPatchelfHook set Nix runtime paths.
     CMAKE_ARGS = "-DBUILD_SHARED_LIBS=ON -DGGML_NATIVE=OFF -DGGML_VULKAN=ON -DCMAKE_POSITION_INDEPENDENT_CODE=ON -DCMAKE_SKIP_RPATH=ON";