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/web-ui.md

@@ -70,7 +70,11 @@ interval (2-600 s) and can be limited to some of the blocks. The first rule that
 page wins. A rule can leave a page chosen by hand alone (untick "also over a page chosen by
 hand"): it then waits until that pick ends (the next block or "Back to schedule"). A rule
 with "show at once in slow mode" ticked brings its page right away in
-[slow mode](#settings), and the display works normally while that page is shown. The display schedule ([Settings](#settings)) still decides when the display is on.
+[slow mode](#settings), and the display works normally while that page is shown.
+
+Pages in a block, and rules that add their page to the rotation, can be left out while slow
+mode is on ("skip in slow mode"). When that leaves none of a block's pages, the chosen page
+is shown instead. The display schedule ([Settings](#settings)) still decides when the display is on.
 
 ![Schedule](../Pictures/web-ui/schedule.png)