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/how-it-works.md

@@ -10,11 +10,14 @@
    connects to MQTT.
 2. **Clock.** Once the time is synced, the ESP32 sends a new frame whenever the screen
    changes, mostly once a minute:
-   - **08:00–22:00:** the page: the chosen page (by default the built-in Clock page,
+   - **08:00–23:30:** the page: the chosen page (by default the built-in Clock page,
      Pikachu with a thought bubble showing `HH:MM`, yawning from 17:00), or what the
      [schedule and rules](web-ui.md#schedule) pick.
-   - **22:00–23:30:** the clock is only shown during the minutes :00, :15, :30 and :45.
-     30 seconds after each of those minutes, the relay turns the display off.
+   - **Slow mode** (a switch in Home Assistant and the web UI, for less noise): the display
+     only changes every 1, 2, 5, 10, 15, 20, 30 or 60 minutes, on the clock (15 minutes =
+     :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.
    - **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