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/runtime_model.h

@@ -50,6 +50,7 @@ struct Tile {
   float battery = NAN, volume = NAN;
   uint32_t supported = 0, background = 0;
   bool transparent = false;  // "Achtergrond: geen": card fill and border hidden, contents unchanged.
+  std::string icon;  // UTF-8 glyph of a chosen icon the icon fonts contain; empty keeps the domain icon.
   std::string revision, pending_revision;
   uint32_t pending_since = 0;
   bool pending = false, confirmed = false, local_feedback = false;