Custom icon per tile: 150 MDI icons in firmware and editor

- screen_manager/app/tile_icons.py is the single icon list (12 groups, 20 media
  and music icons); tools/generate_icons.py writes it into the three icon fonts
  of both boards and builds the editor's subset font.
- Firmware 0.2.18 shows o.icon when its fonts carry the glyph, else the domain
  icon. The app sends the chosen icon, or Home Assistant's own mdi icon on auto.
- Tile sheet gets an icon picker with search; the mockup shows the icon the
  screen actually draws.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

components/smart_display/tile_icon.h

@@ -0,0 +1,25 @@
+#pragma once
+#include <cstdint>
+#include <string>
+
+namespace tile_icon {
+// ESP Screen Manager sends a chosen icon as its Material Design Icons codepoint
+// in hex ("F06B5"). Anything else is 0: the tile keeps its domain icon.
+inline uint32_t codepoint(const std::string &hex) {
+  if (hex.size() != 5) return 0;
+  uint32_t value = 0;
+  for (char c : hex) {
+    int digit = c >= '0' && c <= '9' ? c - '0' : c >= 'A' && c <= 'F' ? c - 'A' + 10 : c >= 'a' && c <= 'f' ? c - 'a' + 10 : -1;
+    if (digit < 0) return 0;
+    value = value << 4 | static_cast<uint32_t>(digit);
+  }
+  // MDI lives in Supplementary Private Use Area-A.
+  return value >= 0xF0000 && value <= 0xFFFFD ? value : 0;
+}
+// Four-byte UTF-8 for a label; empty for 0.
+inline std::string utf8(uint32_t cp) {
+  if (cp < 0x10000 || cp > 0x10FFFF) return {};
+  return {static_cast<char>(0xF0 | cp >> 18), static_cast<char>(0x80 | (cp >> 12 & 0x3F)),
+          static_cast<char>(0x80 | (cp >> 6 & 0x3F)), static_cast<char>(0x80 | (cp & 0x3F))};
+}
+}  // namespace tile_icon