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/app.h

@@ -317,6 +317,7 @@ public:
     bool wantsScreenshotThisFrame() const { return sweep_capture_this_frame_; }
     const std::string& screenshotSweepPath() const { return sweep_current_path_; }
     void onScreenshotCaptured();
+    std::string screenshotDir() const;   // <config>/screenshots (fixed; sweeps overwrite in place)
 
     // Dialog triggers (used by settings page to open modal dialogs)
     void showImportKeyDialog() { show_import_key_ = true; }