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/logic/icon_format.cpp

@@ -0,0 +1,103 @@
+#include "logic/icon_format.h"
+
+#include <string.h>
+
+bool validIconSize(int width, int height)
+{
+	return width >= 1 && height >= 1 && width <= ICON_MAX_WIDTH && height <= ICON_MAX_HEIGHT;
+}
+
+bool iconPixel(const Icon &icon, int x, int y)
+{
+	if (x < 0 || y < 0 || x >= icon.width || y >= icon.height)
+		return false;
+	return icon.data[y * iconRowBytes(icon.width) + x / 8] & (0x80 >> (x & 7));
+}
+
+void setIconPixel(Icon &icon, int x, int y, bool on)
+{
+	if (x < 0 || y < 0 || x >= icon.width || y >= icon.height)
+		return;
+	uint8_t &byte = icon.data[y * iconRowBytes(icon.width) + x / 8];
+	if (on)
+		byte |= 0x80 >> (x & 7);
+	else
+		byte &= ~(0x80 >> (x & 7));
+}
+
+static int hexValue(char c)
+{
+	if (c >= '0' && c <= '9')
+		return c - '0';
+	if (c >= 'a' && c <= 'f')
+		return c - 'a' + 10;
+	if (c >= 'A' && c <= 'F')
+		return c - 'A' + 10;
+	return -1;
+}
+
+bool iconFromHex(int width, int height, const char *hex, Icon &icon)
+{
+	if (!validIconSize(width, height) || hex == nullptr)
+		return false;
+	int bytes = iconBytes(width, height);
+	if (strlen(hex) != (size_t)bytes * 2)
+		return false;
+	Icon result;
+	result.width = width;
+	result.height = height;
+	for (int i = 0; i < bytes; i++)
+	{
+		int high = hexValue(hex[2 * i]);
+		int low = hexValue(hex[2 * i + 1]);
+		if (high < 0 || low < 0)
+			return false;
+		result.data[i] = (high << 4) | low;
+	}
+	// The unused bits at the end of each row stay off
+	for (int y = 0; y < height; y++)
+		for (int x = width; x < iconRowBytes(width) * 8; x++)
+			result.data[y * iconRowBytes(width) + x / 8] &= ~(0x80 >> (x & 7));
+	icon = result;
+	return true;
+}
+
+void iconToHex(const Icon &icon, char *out)
+{
+	static const char digits[] = "0123456789abcdef";
+	int bytes = iconBytes(icon.width, icon.height);
+	for (int i = 0; i < bytes; i++)
+	{
+		out[2 * i] = digits[icon.data[i] >> 4];
+		out[2 * i + 1] = digits[icon.data[i] & 0x0F];
+	}
+	out[bytes * 2] = '\0';
+}
+
+void iconToBitmap(const Icon &icon, uint8_t *bitmap)
+{
+	memset(bitmap, 0, BITMAP_BYTES);
+	for (int y = 0; y < icon.height; y++)
+		for (int x = 0; x < icon.width; x++)
+			if (iconPixel(icon, x, y))
+				setBitmapPixel(bitmap, x, y, true);
+}
+
+void iconFromBitmap(const uint8_t *bitmap, Icon &icon)
+{
+	int right = 0, bottom = 0;
+	for (int y = 0; y < BITMAP_HEIGHT; y++)
+		for (int x = 0; x < BITMAP_WIDTH; x++)
+			if (bitmapPixel(bitmap, x, y))
+			{
+				right = x > right ? x : right;
+				bottom = y > bottom ? y : bottom;
+			}
+	icon = Icon();
+	icon.width = right + 1;
+	icon.height = bottom + 1;
+	for (int y = 0; y < icon.height; y++)
+		for (int x = 0; x < icon.width; x++)
+			if (bitmapPixel(bitmap, x, y))
+				setIconPixel(icon, x, y, true);
+}