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/display/bitmap_store.cpp

@@ -51,7 +51,7 @@ static String path(const char *name)
 bool bitmapStoreBegin()
 {
 	mounted = LittleFS.begin(true);
-	for (const char *dir : {DIR, "/pages", "/fonts"})
+	for (const char *dir : {DIR, "/pages", "/fonts", "/icons"})
 		if (mounted && !LittleFS.exists(dir))
 			LittleFS.mkdir(dir);
 	Serial.printf("Bitmap storage %s\n", mounted ? "ready" : "not available (old flash layout?)");
@@ -108,6 +108,11 @@ uint32_t bitmapListRevision()
 	return revision;
 }
 
+bool builtinBitmap(const char *name)
+{
+	return builtin(name) != nullptr;
+}
+
 bool bitmapExists(const char *name)
 {
 	if (builtin(name))