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/app.h

@@ -31,6 +31,11 @@ void appLoadSettings();
 // restart_needed when the change only works after a restart (Home Assistant MQTT).
 const char *appSaveSettings(const Settings &settings, bool &restart_needed);
 
+// Slow mode (Home Assistant switch and dropdown): stored and used right away. minutes: one
+// of SLOW_MODE_MINUTES. Return nullptr or an error message.
+void appSetSlowMode(bool enabled);
+const char *appSetSlowModeMinutes(int minutes);
+
 // The chosen page (settings): a dynamic page or a bitmap. With a schedule it is shown until
 // the next block starts (a manual pick), without one all the time.
 const char *appSelectPage(const char *name);
@@ -63,6 +68,12 @@ const char *appIconToBitmap(const char *name);
 // Once: the uploaded bitmaps that only pages use become icons. Call from loop(); returns at
 // once when it is done.
 void appMigrateIcons();
+// Once per boot, after the firmware was marked good (the old firmware can't read the new
+// formats): settings and schedule read in an older layout are stored again, and pages in an
+// older format are rewritten. Call from loop(); returns at once when it is done.
+void appMigrateData();
+// Records still in an older format (-1 until appMigrateData() checked the pages)
+int appOldDataLeft();
 const char *appSavePage(const char *name, const DynamicPage &page);
 const char *appDeletePage(const char *name);
 const char *appSaveFont(const char *name, const uint8_t *data, size_t size);