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

tests/test_faster_whisper_engine.py

@@ -326,17 +326,17 @@ class TestFasterWhisperEngine:
 
     def test_transcribe_passes_initial_prompt(self) -> None:
         """Test that vocabulary bias is forwarded to faster-whisper."""
-        whisper_mock = self._mock_whisper_model([MagicMock(text="VocaLinux")])
+        whisper_mock = self._mock_whisper_model([MagicMock(text="Voculinux")])
         with patch.dict(sys.modules, {"faster_whisper": whisper_mock}):
             engine = FasterWhisperEngine(model_size="tiny", device="cpu")
             engine.init()
 
             audio = np.array([0, 1000, -1000, 0], dtype=np.int16)
-            engine.transcribe([audio.tobytes()], initial_prompt="VocaLinux")
+            engine.transcribe([audio.tobytes()], initial_prompt="Voculinux")
 
             assert (
                 whisper_mock.WhisperModel.return_value.transcribe.call_args.kwargs["initial_prompt"]
-                == "VocaLinux"
+                == "Voculinux"
             )
 
     def test_transcribe_empty_audio(self):
@@ -429,7 +429,7 @@ class TestFasterWhisperEngine:
             assert text == ""
 
     def test_language_mapping(self):
-        """Test that Vocalinux language codes are mapped correctly."""
+        """Test that Voculinux language codes are mapped correctly."""
         engine = FasterWhisperEngine(language="en-us")
         assert engine._normalize_language() == "en"