feat(recipes): add recently opened sort with modal open tracking

Track recipe modal opens in a separate stats file (never touching recipe
JSON/EXIF), expose a fire-and-forget POST endpoint, and add an 'opened'
sort that hides never-opened recipes as a true recently-opened view.
Includes i18n for all locales and backend/frontend tests.

tests/frontend/utils/masonryScroller.test.js

@@ -324,6 +324,18 @@ describe('MasonryScroller', () => {
     expect(placeholder.textContent).toContain('No recipes found');
   });
 
+  it('shows the recently-opened empty placeholder under the opened sort', async () => {
+    getCurrentPageState().sortBy = 'opened:desc';
+    const { scroller, grid } = track(createScroller({ items: [] }));
+
+    await scroller.initialize();
+
+    const placeholder = grid.querySelector('#virtualScrollPlaceholder');
+    expect(placeholder).not.toBeNull();
+    expect(placeholder.textContent).toContain('No recently opened recipes');
+    getCurrentPageState().sortBy = '';
+  });
+
   it('dispose removes classes, spacer and event listeners', () => {
     const { scroller, grid } = track(createScroller());