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>

tests/test_tile_icon.cpp

@@ -0,0 +1,12 @@
+#include "components/smart_display/tile_icon.h"
+#include <cassert>
+int main() {
+  assert(tile_icon::codepoint("F06B5")==0xF06B5);
+  assert(tile_icon::codepoint("f1d17")==0xF1D17);
+  // Names, short or long hex, non-MDI ranges and junk keep the domain icon.
+  for (const char *bad : {"", "lamp", "auto", "F06B", "F06B50", "006B5", "F06G5", " F06B", "-F06B"})
+    assert(tile_icon::codepoint(bad)==0);
+  assert(tile_icon::utf8(0xF06B5)=="\U000F06B5");
+  assert(tile_icon::utf8(0xF0335)=="\U000F0335");
+  assert(tile_icon::utf8(0).empty() && tile_icon::utf8(0x41).empty());
+}