feat(ui-lvgl): screenshots (side + top button), lock screen always times out

- Side button held + top button saves the screen, popups and toasts included,
  to /sdcard/screenshots/scr_NNNN.bmp (24-bit BMP). The flush callbacks copy
  one forced full re-render into a PSRAM buffer. A white flash and a toast
  confirm it. The side button's mute now fires on release, so holding it a
  little long before the top button doesn't also mute. In the sim, the page's
  Screenshot button downloads the file.
- The lock screen goes dark after 30 s even with auto-off at Never (a boot
  PIN, a pocket wake-up, a message), and sooner if auto-off is shorter.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

examples/companion_radio/ui-lvgl/UITask.h

@@ -401,6 +401,7 @@ private:
   void wake();
   void sleep();
   void toggleMute();
+  void takeScreenshot();
   uint32_t _next_wake_poll_ms = 0;
   uint32_t _next_touch_poll_ms = 0;   // tap to wake, while the screen is off
   bool _wake_down = false;