Slow mode replaces the late-evening quarter hours

Slow mode (Home Assistant switch "FlipDot Display Slow mode", dropdown
"FlipDot Display Slow mode interval", web UI Settings) makes the display
change only every 1, 2, 5, 10, 15, 20, 30 or 60 minutes, on the clock, and
powers it off in between: less noise. Nothing else is drawn meanwhile, also
no page changes. It updates at once after boot, the night, the Power switch
and live mode, and when it is switched off. Rules can override it ("show at
once in slow mode"): while their page is shown the display works normally.

Settings layout 7 and schedule layout 4 (older records are converted; they
are stored again once the firmware is marked good, so a rollback still
finds the old ones). Pages in older formats are rewritten in the current
one, in time-boxed steps. /api/status reports old_data_left.

Version 2.15.0.

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

docs/web-ui.md

@@ -66,7 +66,9 @@ Rules bring a page to the front from a Home Assistant state (an entity or attrib
 while it is true, or show it for a while when it becomes true. Each rule is read at its own
 interval (2-600 s) and can be limited to some of the blocks. The first rule that shows a
 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"). The display schedule ([Settings](#settings)) still decides when the display is on.
+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.
 
 ![Schedule](../Pictures/web-ui/schedule.png)
 
@@ -114,8 +116,9 @@ on the dots. Save it as a bitmap with a name, or as an icon (only the area with
 
 ## Settings
 
-The display schedule (on/off, when the display is on, the late-evening quarter-hour mode,
-when it switches off); the "No WiFi" screen; the flip time, the pause after each pulse,
+The display schedule (on/off, when the display is on, when it switches off); slow mode
+(also a Home Assistant switch: the display only changes every 1-60 minutes, on the clock,
+and is switched off in between, for less noise); the "No WiFi" screen; the flip time, the pause after each pulse,
 pulses per dot and the refresh effect (instant, sparkle, wipe, rain, from the middle out,
 diagonal, spiral, flip all then reveal, push, random; optionally only for small changes
 like the clock's minutes; pages can choose their own effect); how often every dot is flipped again ("Dot refresh", default