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>

CLAUDE.md

@@ -98,9 +98,15 @@ See README.md for the overview.
 - All settings are in `config.h`. Keep it free of Arduino types for the same reason.
 - Frames are only sent when the screen changes. The schedule uses 08:00 as the start of
   the day on purpose (that is what the old code did, and the README documents it).
+- Slow mode (`SlowModeSettings`, replaced the late-evening quarter hours) lives in
+  `DisplayController::_slowClock()`: one frame per update minute (`slowModeUpdate()`), power
+  off in between, nothing else drawn (also no page changes). It updates at once after boot,
+  night, the Power switch and live mode; rules with `over_slow_mode` suspend it
+  (`setSlowModeOverride()`, fed from `PageChoice` in `appPageLoop()`).
 - Home Assistant switches are not optimistic: callbacks must call `sender->setState()`.
   The unique ids (`FlipDotDisplayPower`, `FlipDotDisplayMode`, `FlipDotDisplayOffImage`,
-  `FlipDotDisplayStopwatch`, ...) and `HA_DEVICE_ID` must stay
+  `FlipDotDisplayStopwatch`, `FlipDotDisplaySlowMode`, `FlipDotDisplaySlowModeInterval`,
+  ...) and `HA_DEVICE_ID` must stay
   the same, or Home Assistant creates new entities.
 
 ## Keeping ESP32 OTA alive (most important requirement)