Icons: small images of their own size for pages

Bitmaps that only pages use (backgrounds, images by state) are now icons:
own width and height, stored in /icons, not in the page lists or the Home
Assistant dropdowns. Page backgrounds and image elements use icons or
bitmaps; the renderer gets icons expanded at the top left, so pages look
the same. Web UI: Icons tab with an editor, Make icon / Make bitmap, Save
as icon on the Draw tab, icons in the backup.

A one-time migration in time-boxed steps moves the uploaded bitmaps that
only pages use, after the new firmware is marked good.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/esp32/logic/page.cpp

@@ -5,6 +5,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <strings.h>
 
 static const uint16_t MINUTES_PER_DAY = 24 * 60;
 static const uint32_t MAX_TARGET = 36525UL * MINUTES_PER_DAY; // 2100-01-01
@@ -70,6 +71,31 @@ bool elementUsesHa(const PageElement &element)
 	}
 }
 
+bool pageUsesImage(const DynamicPage &page, const char *name, bool by_state)
+{
+	for (int i = 0; i < page.slot_count; i++)
+		if (sameName(page.slots[i].bitmap, name))
+			return true;
+	for (int i = 0; i < page.element_count; i++)
+	{
+		const PageElement &e = page.elements[i];
+		if (e.type != ElementType::Image)
+			continue;
+		if (!e.entity[0])
+		{
+			if (sameName(e.text, name))
+				return true;
+			continue;
+		}
+		size_t start = strlen(e.text);
+		if (strncasecmp(name, e.text, start) != 0)
+			continue;
+		if (by_state ? name[start] != '\0' : strcasecmp(name + start, "default") == 0)
+			return true;
+	}
+	return false;
+}
+
 bool validEntityId(const char *entity)
 {
 	if (entity == nullptr)