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.h

@@ -31,6 +31,8 @@ std::vector<BitmapInfo> listBitmaps();
 // Changes each time a bitmap is saved or deleted
 uint32_t bitmapListRevision();
 bool bitmapExists(const char *name);
+// Without reading the storage
+bool builtinBitmap(const char *name);
 bool loadBitmap(const char *name, uint8_t *bitmap);
 
 // Return nullptr on success, otherwise a message for the user