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>

src/esp32/logic/settings.h

@@ -14,11 +14,21 @@ struct ScheduleSettings
 {
 	bool enabled = true;            // false: the display is always on
 	uint16_t day_start = 8 * 60;    // display on from
-	uint16_t late_start = 22 * 60;  // from here only at the quarter hours (see quarter_hours)
-	bool quarter_hours = true;      // false: on all the time until night_start
 	uint16_t night_start = 23 * 60 + 31; // display off from (up to 24:00 = never)
 };
 
+// Slow mode (Home Assistant switch "FlipDot Display Slow mode", web UI): the display only
+// changes every `minutes` minutes, on the clock (15: at :00 :15 :30 :45), and is powered
+// off in between. Fewer flips, so less noise.
+const uint8_t SLOW_MODE_MINUTES[] = {1, 2, 5, 10, 15, 20, 30, 60}; // all divide an hour
+const int SLOW_MODE_MINUTES_COUNT = sizeof(SLOW_MODE_MINUTES) / sizeof(SLOW_MODE_MINUTES[0]);
+struct SlowModeSettings
+{
+	bool enabled = false;
+	uint8_t minutes = 15;           // one of SLOW_MODE_MINUTES
+};
+bool validSlowModeMinutes(int minutes);
+
 struct ClockSettings
 {
 	bool no_wifi_screen = true;     // "No WiFi" instead of a dynamic page when WiFi is down
@@ -85,6 +95,7 @@ struct Settings
 	RefreshSettings refresh;
 	// Plex user name for {plex_user} in entity ids (Tautulli's sensors of that user)
 	char plex_user[PLEX_USER_SIZE] = "";
+	SlowModeSettings slow_mode;
 	// New fields go here, at the end (see system/settings_store.cpp)
 };