feat(rembg): add RemoveBG Studio - native background removal app

C++ WebView2 host app driving a Python/rembg engine as a JSON-stdio
sidecar, with live before/after preview for images and video frames.
Also includes the original CLI (removebg.py) built on the same
core/ engine.

The WebView2 SDK is fetched on demand via
native/third_party/fetch-webview2.ps1 (wired into the Makefile)
rather than vendored, since it's ~15MB of prebuilt/generated
Microsoft SDK content. A Makefile provides make sync/build/run/clean
as the standard entry points.

rembg/Makefile

@@ -0,0 +1,39 @@
+# RemoveBG Studio
+#
+# Common entry points for building/running the native app and keeping the
+# Python engine's virtualenv in sync. Requires: CMake, MSVC (Visual Studio
+# 2022 Build Tools or full VS), PowerShell, and uv (or pip) for Python deps.
+
+GENERATOR ?= Visual Studio 17 2022
+CONFIG ?= Release
+BUILD_DIR := native/build
+WEBVIEW2_HEADER := native/third_party/webview2/include/WebView2.h
+EXE := $(BUILD_DIR)/$(CONFIG)/RemoveBGStudio.exe
+
+.PHONY: all sync fetch-deps configure build run clean
+
+all: build
+
+# --- Python engine -----------------------------------------------------
+
+sync:
+	uv sync
+
+# --- Native app ----------------------------------------------------------
+
+$(WEBVIEW2_HEADER):
+	powershell -ExecutionPolicy Bypass -File native/third_party/fetch-webview2.ps1
+
+fetch-deps: $(WEBVIEW2_HEADER)
+
+configure: fetch-deps
+	cmake -S native -B $(BUILD_DIR) -G "$(GENERATOR)" -A x64
+
+build: configure
+	cmake --build $(BUILD_DIR) --config $(CONFIG)
+
+run: build
+	$(EXE)
+
+clean:
+	rm -rf $(BUILD_DIR)