Schedule pages and rotation rules can be skipped in slow mode

Each page in a schedule block, and each rule that adds its page to the
rotation, has a "skip in slow mode" checkbox. While slow mode is on those
pages are left out of the rotation; when none of the block's pages are
left, the chosen page is shown. The rotation keeps its own timers.

Schedule format version 5; version 4 is converted when read and stored
again in version 5 on the next save.

Version 2.18.0.

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

docs/how-it-works.md

@@ -26,6 +26,8 @@
      :00, :15, :30 and :45). 30 seconds after each update the relay turns the display off;
      the dots keep the image. Page changes wait for the next update too, except for rules
      with "show at once in slow mode": while their page is shown, the display works normally.
+     Schedule pages and rotation rules with "skip in slow mode" are left out of the rotation
+     (none left in the block: the chosen page).
    - **23:31–08:00:** the screen is cleared (or shows the off image chosen in the web UI)
      and power is cut after 30 seconds.
 3. **Home Assistant**: switches, dropdowns and sensors over MQTT, see