feat(debug): theme x tab screenshot sweep + Debug options dialog

Add a debug tool that cycles every skin across every build-enabled tab and saves a
PNG of each to a timestamped folder under the config dir (screenshots/sweep_<ts>/)
— for gathering visual context on theme-specific UI work.

- App state machine (app_network.cpp): startScreenshotSweep() builds the
  skin+enabled-page lists, saves the current skin/page, and steps through them;
  updateScreenshotSweep() (top of App::render) pins the page + settles ~4 frames
  after each skin/page change (skin switches reload TOML + reset the acrylic
  capture, so they need to settle); wantsScreenshotThisFrame()/screenshotSweepPath()
  /onScreenshotCaptured() coordinate with main.cpp. Restores the original skin/page
  when done; nothing persisted.
- Framebuffer capture (main.cpp): read the finished frame and encode PNG via the
  bundled miniz (tdefl_write_image_to_png_file_in_memory_ex). GL reads FBO 0 (RGBA,
  bottom-up -> flip); DX11 copies the backbuffer to a staging texture + maps it
  (BGRA, top-down -> channel-swap). Alpha forced opaque.
- Settings UI: move the Verbose logging checkbox off the wallet row into a new
  "Debug options" button that opens a dialog housing the verbose toggle + a "Run
  screenshot sweep" button. New i18n keys.

Both platforms build; no-crash smoke verified.

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

src/util/i18n.cpp

@@ -253,6 +253,10 @@ void I18n::loadBuiltinEnglish()
         strings_["keep_daemon"] = "Keep daemon running";
         strings_["stop_external"] = "Stop external daemon";
         strings_["verbose_logging"] = "Verbose logging";
+        strings_["debug_options"] = "Debug options\xE2\x80\xA6";
+        strings_["debug_options_title"] = "Debug options";
+        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_["mine_when_idle"] = "Mine when idle";
         strings_["setup_wizard"] = "Run Setup Wizard...";