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/util/i18n.cpp

@@ -254,7 +254,8 @@ void I18n::loadBuiltinEnglish()
         strings_["stop_external"] = "Stop external daemon";
         strings_["verbose_logging"] = "Verbose logging";
         strings_["screenshot_sweep"] = "Run screenshot sweep";
-        strings_["screenshot_sweep_desc"] = "Cycles every theme across every tab and saves a screenshot of each to a timestamped folder in the config directory. Runs for a few seconds; the path is shown when it finishes.";
+        strings_["screenshot_open_dir"] = "Open location";
+        strings_["screenshot_sweep_desc"] = "Cycles every theme across every tab and saves a screenshot of each into per-tab subfolders under the config directory's screenshots folder (overwriting the previous sweep). Runs for a few seconds.";
         strings_["mine_when_idle"] = "Mine when idle";
         strings_["setup_wizard"] = "Run Setup Wizard...";