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/app.h

@@ -7,6 +7,7 @@
 #include "display/frames.h"
 #include "display/status_screen.h"
 #include "logic/display_controller.h"
+#include "logic/icon_format.h"
 #include "logic/local_time.h"
 #include "logic/loop_stats.h"
 #include "logic/page.h"
@@ -48,11 +49,20 @@ void appPageLoop(bool time_valid, const LocalTime &time);
 // Left on the dots when the display switches off: OFF_PAGE_BLANK or a bitmap name
 const char *appSelectOffPage(const char *name);
 
-// Bitmaps, dynamic pages and fonts: stored, and the display redrawn when it shows them.
-// Bitmaps and pages share one namespace; what a page uses can't be deleted. All return
-// nullptr or an error message.
+// Bitmaps, icons, dynamic pages and fonts: stored, and the display redrawn when it shows
+// them. Bitmaps, icons and pages share one namespace; what a page uses can't be deleted.
+// All return nullptr or an error message.
 const char *appSaveBitmap(const char *name, const uint8_t *bitmap);
 const char *appDeleteBitmap(const char *name);
+const char *appSaveIcon(const char *name, const Icon &icon);
+const char *appDeleteIcon(const char *name);
+// A bitmap becomes an icon (cut to its dots from the top left, so pages look the same) and
+// the other way around; not for bitmaps that are shown on their own
+const char *appBitmapToIcon(const char *name);
+const char *appIconToBitmap(const char *name);
+// Once: the uploaded bitmaps that only pages use become icons. Call from loop(); returns at
+// once when it is done.
+void appMigrateIcons();
 const char *appSavePage(const char *name, const DynamicPage &page);
 const char *appDeletePage(const char *name);
 const char *appSaveFont(const char *name, const uint8_t *data, size_t size);