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/main.cpp

@@ -81,6 +81,9 @@ void loop()
 	homeAssistantLoop();
 	webUiLoop();
 	appLoop();
+	// Once the firmware can't be rolled back anymore (the old one doesn't know icons)
+	if (!firmwareGuardPending())
+		appMigrateIcons();
 	// Home Assistant values of the shown page (read by a task, never blocks)
 	if (haApiLoop(wifiConnected()))
 		controller.pageChanged();