build(freetype): color-emoji rendering via a cross-platform FreeType backend

The chat "Emoji style: Color" option renders the merged emoji in color (COLR/CPAL
Twemoji) instead of the monochrome NotoEmoji subset. This needs FreeType, which the
default stb_truetype rasterizer can't do for color glyphs.

- Vendor imgui_freetype (matches the bundled 1.92 ImFontLoader API) and embed a 1.4 MB
  COLRv0 Twemoji font (no libpng/harfbuzz needed).
- CMake gains an optional FreeType path: native Linux/macOS use the system FreeType via
  find_package; the mingw-w64 cross-compile has none, so build.sh --win-release now
  cross-builds a minimal static FreeType (scripts/build-freetype-mingw.sh) and passes it
  in. Absent FreeType => graceful monochrome fallback, so no build breaks.
- Typography selects the FreeType loader + the color font (LoadColor) when color emoji is
  on, else the stb loader + mono subset; toggling reloads the atlas. Both the DX11 and
  OpenGL backends already support the 1.92 RGBA dynamic atlas, so color glyphs render.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/embedded/embedded_fonts.cpp.in

@@ -16,3 +16,4 @@ INCBIN(material_icons,  "@CMAKE_SOURCE_DIR@/res/fonts/MaterialIcons-Regular.ttf"
 INCBIN(mdi_pickaxe_subset, "@CMAKE_SOURCE_DIR@/res/fonts/MaterialDesignIcons-Pickaxe-Subset.ttf");
 INCBIN(noto_cjk_subset, "@CMAKE_SOURCE_DIR@/res/fonts/NotoSansCJK-Subset.ttf");
 INCBIN(noto_emoji_subset, "@CMAKE_SOURCE_DIR@/res/fonts/NotoEmoji-Subset.ttf");
+INCBIN(twemoji_color, "@CMAKE_SOURCE_DIR@/res/fonts/TwemojiMozilla-Color.ttf");

src/embedded/embedded_fonts.h

@@ -38,4 +38,8 @@ extern "C" {
 
     extern const unsigned char g_noto_emoji_subset_data[];
     extern const unsigned int  g_noto_emoji_subset_size;
+
+    // Twemoji COLR/CPAL color-emoji font (used only when color emoji is enabled + FreeType is available).
+    extern const unsigned char g_twemoji_color_data[];
+    extern const unsigned int  g_twemoji_color_size;
 }