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>

.gitignore

@@ -54,3 +54,7 @@ third_party/silentdragonxlite/lib/vendor/
 
 # Generated by configure_file from res/ObsidianDragon.manifest.in (do not track)
 res/ObsidianDragon.manifest
+
+# Cross-built mingw FreeType (color emoji) — regenerated by scripts/build-freetype-mingw.sh
+third_party/freetype-mingw/
+third_party/.freetype-mingw-build/