fix(identity): share one model-download user agent definition

Both model-download call sites built the User-Agent string separately:
recognition_manager used a hardcoded literal while settings_dialog
interpolated APP_ID. Add app_identity.user_agent() and use it in both,
with a test pinning the shared definition.

Fixes #17

tests/test_app_identity.py

@@ -117,6 +117,20 @@ def test_source_pinned_literals_reserved_for_rename():
     assert 'window.set_title("Voculinux Dictation Pad")' in pad
 
 
+def test_model_download_user_agent_is_shared_definition():
+    """Model-download User-Agent strings resolve through one shared helper."""
+    identity = _load_identity()
+    assert identity.user_agent("9.9.9") == f"{identity.APP_ID}/9.9.9"
+    for rel in (
+        SRC_ROOT / "speech_recognition" / "recognition_manager.py",
+        SRC_ROOT / "ui" / "settings_dialog.py",
+    ):
+        source = rel.read_text(encoding="utf-8")
+        assert "user_agent(__version__)" in source, rel
+        assert 'f"voculinux/{__version__}"' not in source, rel
+        assert 'f"{APP_ID}/{__version__}"' not in source, rel
+
+
 def test_rename_inventory_files_exist():
     """Every rename surface the rename ticket must flip is a real checked file."""
     identity = _load_identity()