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>

tools/make_webui_screenshots.py

@@ -33,6 +33,7 @@ SHOTS = [
     ('schedule', 'schedule', 1150),
     ('stopwatch', 'stopwatch', 560),
     ('bitmaps', 'bitmaps', 1600),
+    ('icons', 'icons', 1900),
     ('draw', 'draw', 820),
     ('settings', 'settings', 1550),
     ('home-assistant', 'ha', 780),