feat(debug): per-tab screenshot subfolders + overwrite + Open location button

- Organize sweep output into per-tab subfolders: <config>/screenshots/<tab>/<skin>.png
  (was one timestamped folder with idx_skin_tab.png names).
- Fixed folder (no timestamp): subsequent sweeps overwrite the existing PNGs in
  place instead of creating a new directory each run.
- Add an "Open location" TactileButton next to "Run screenshot sweep" that opens
  the screenshots folder via Platform::openFolder(app->screenshotDir()).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/ui/pages/settings_page.cpp

@@ -2463,6 +2463,9 @@ void RenderSettingsPage(App* app) {
             ImGui::Dummy(ImVec2(0, Layout::spacingXs()));
             if (TactileButton(TR("screenshot_sweep"), ImVec2(0, 0), S.resolveFont("button")))
                 app->startScreenshotSweep();
+            ImGui::SameLine();
+            if (TactileButton(TR("screenshot_open_dir"), ImVec2(0, 0), S.resolveFont("button")))
+                util::Platform::openFolder(app->screenshotDir());
             ImGui::Dummy(ImVec2(0, Layout::spacingSm()));
             ImGui::Separator();
             ImGui::Dummy(ImVec2(0, Layout::spacingSm()));