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>

build.sh

@@ -725,12 +725,27 @@ HDR
         "$SCRIPT_DIR/scripts/fetch-libsodium.sh" --win
     fi
 
+    # ── FreeType for Windows (color-emoji rendering) ───────────────────────
+    # The mingw toolchain ships no FreeType; cross-build a minimal static one (COLR/CPAL, no external
+    # deps). Failure is non-fatal — the wallet just falls back to monochrome emoji.
+    local FT_MINGW_PREFIX="$SCRIPT_DIR/third_party/freetype-mingw"
+    if [[ ! -f "$FT_MINGW_PREFIX/lib/libfreetype.a" ]]; then
+        info "Cross-building FreeType for Windows (color emoji) ..."
+        "$SCRIPT_DIR/scripts/build-freetype-mingw.sh" "$FT_MINGW_PREFIX" \
+            || warn "FreeType cross-build failed — Windows build will use monochrome emoji"
+    fi
+    local FT_CMAKE_ARG=()
+    if [[ -f "$FT_MINGW_PREFIX/lib/libfreetype.a" ]]; then
+        FT_CMAKE_ARG=(-DDRAGONX_MINGW_FREETYPE_PREFIX="$FT_MINGW_PREFIX")
+    fi
+
     # ── CMake + build ────────────────────────────────────────────────────────
     info "Configuring (cross-compile) ..."
     cmake "$SCRIPT_DIR" \
         -DCMAKE_TOOLCHAIN_FILE="$bd/mingw-toolchain.cmake" \
         -DCMAKE_BUILD_TYPE=Release \
         -DDRAGONX_USE_SYSTEM_SDL3=OFF \
+        "${FT_CMAKE_ARG[@]}" \
         "${CMAKE_LITE_ARGS[@]}"
 
     info "Building with $JOBS jobs ..."