Add screen-timeout slider screen; move screensaver toggle into it

Replaces the repeated-tap timeout cycling (0-10, one tap per minute)
with a dedicated screen and slider (1-15 minutes, then "Never"). The
screensaver toggle moves from its own System-menu row into this
screen, directly below the slider, since it only determines what
happens when the timeout fires.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/i18n_en.h

@@ -254,6 +254,8 @@ static const char* const I18N_EN[] = {
     "The new firmware was installed successfully. Please restart now to start using it.",
     "Restart now",
     "The update could not be downloaded or installed. The device is still running the previous firmware, unchanged. Please check your WiFi connection and try again later.",
+    "Screen Timeout",
+    "After this time without touch, the display turns off to save power. With the screensaver on, it dims to a starfield with a clock instead.",
 };
 
 static_assert(sizeof(I18N_EN) / sizeof(I18N_EN[0]) == (size_t)StringId::COUNT,