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/display/page_store.cpp

@@ -536,6 +536,23 @@ bool loadPage(const char *name, DynamicPage &page)
 	return ok || builtinPage(name, page);
 }
 
+std::vector<String> storedPageNames()
+{
+	return storedNames(DIR, EXTENSION);
+}
+
+bool pageInOldFormat(const char *name)
+{
+	if (!storedPage(name))
+		return false;
+	File file = LittleFS.open(path(name), "r");
+	if (!file)
+		return false;
+	bool old = file.size() != sizeof(StoredPage);
+	file.close();
+	return old;
+}
+
 const char *savePage(const char *name, const DynamicPage &page)
 {
 	if (!validBitmapName(name))

src/esp32/display/page_store.h

@@ -31,6 +31,10 @@ bool pageExists(const char *name);
 // A built-in page that no stored page replaces
 bool builtinPageExists(const char *name);
 bool loadPage(const char *name, DynamicPage &page);
+// The stored pages (slow: lists the directory) and whether one is stored in an older
+// format (loadPage() converts it, savePage() writes the current one)
+std::vector<String> storedPageNames();
+bool pageInOldFormat(const char *name);
 
 // Return nullptr on success, otherwise a message for the user. savePage() validates the
 // page, but does not check that its bitmaps and fonts exist.