feat: implement Save As (was stubbed)

Replaces the "coming soon" lambda with _on_action_save_as(), which opens
QFileDialog.getSaveFileName, sets the filename field, and delegates to
_on_save_clicked(). Adds 2 integration tests; also wires mock_file_svc.save_file
to the real FileService so file-content assertions work.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: chrisdpurcell

src/views/main_window.py

@@ -221,9 +221,7 @@ def _connect_signals(self) -> None:
         self._action_quit.triggered.connect(QApplication.quit)
         self._action_save.triggered.connect(self._on_save_clicked)
         self._action_open.triggered.connect(self._on_action_open)
-        self._action_save_as.triggered.connect(
-            lambda: self.ui.statusBar().showMessage("Save As — coming soon")
-        )
+        self._action_save_as.triggered.connect(self._on_action_save_as)
         self._action_about.triggered.connect(self._on_action_about)
         self._action_preferences.triggered.connect(
             lambda: self.ui.statusBar().showMessage("Preferences — coming soon")
@@ -359,6 +357,20 @@ def _on_action_open(self) -> None:
             self._file_name_edit.setText(path)
             self._viewmodel.load_file(path)
 
+    def _on_action_save_as(self) -> None:
+        """Open a Save As dialog; save to the chosen path if confirmed."""
+        _glob = " ".join(TEXT_FILE_EXTENSIONS)
+        initial = self._file_name_edit.text() or QDir.homePath()
+        path, _ = QFileDialog.getSaveFileName(
+            self.ui,
+            "Save As",
+            initial,
+            f"Text Files ({_glob});;All Files (*)",
+        )
+        if path:
+            self._file_name_edit.setText(path)
+            self._on_save_clicked()
+
     def _on_action_about(self) -> None:
         """Show an About dialog."""
         QMessageBox.about(

tests/integration/test_main_window.py

@@ -14,10 +14,17 @@
 
 @pytest.fixture
 def mock_file_svc():
+    from src.services.file_service import FileService
+
     svc = MagicMock()
     svc.open_file.return_value = TextDocument(
         filepath="/tmp/test.txt", content="hello world", encoding="utf-8"
     )
+    # Delegate save_file to the real FileService so tests that write files to
+    # tmp_path can verify disk contents. Tests that only check signals are
+    # unaffected because they don't inspect file system state.
+    _real_svc = FileService()
+    svc.save_file.side_effect = _real_svc.save_file
     return svc
 
 
@@ -453,3 +460,29 @@ def test_char_count_updates_on_delete_without_cursor_move(self, window, qtbot):
             cursor.deleteChar()
         qtbot.wait(10)
         assert "8 chars" in window._cursor_label.text()
+
+
+class TestActionSaveAsHandler:
+    def test_chosen_path_is_saved(self, window, tmp_path, monkeypatch, qtbot):
+        """getSaveFileName returns path → file saved to that path."""
+        out = tmp_path / "renamed.txt"
+        monkeypatch.setattr(
+            "src.views.main_window.QFileDialog.getSaveFileName",
+            lambda *a, **kw: (str(out), ""),
+        )
+        window._plain_text_edit.setPlainText("save-as content")
+        with qtbot.waitSignal(window._viewmodel.file_saved, timeout=2000):
+            window._on_action_save_as()
+        assert out.read_text(encoding="utf-8") == "save-as content"
+        assert window._file_name_edit.text() == str(out)
+
+    def test_cancelled_dialog_is_no_op(self, window, monkeypatch):
+        """getSaveFileName returns '' → nothing saved."""
+        monkeypatch.setattr(
+            "src.views.main_window.QFileDialog.getSaveFileName",
+            lambda *a, **kw: ("", ""),
+        )
+        emitted: list = []
+        window._viewmodel.file_saved.connect(emitted.append)
+        window._on_action_save_as()
+        assert emitted == []