chore: add Windows cross-build script

scripts/build-windows-exe.sh

@@ -0,0 +1,65 @@
+#!/usr/bin/env bash
+# Cross-compile the Windows x64 executable from Linux for local testing.
+# Produces the same layout as the release workflow's portable ZIP:
+#   target/windows/flectar-mail/          (run flectar-mail.exe from here)
+#   target/windows/flectar-mail-windows-x64.zip
+#
+# One-time setup:
+#   sudo apt install clang lld llvm
+#   rustup target add x86_64-pc-windows-msvc
+#   cargo install --locked cargo-xwin
+# cargo-xwin downloads the Microsoft CRT and Windows SDK on first use; doing so
+# accepts Microsoft's license terms for those files.
+set -euo pipefail
+
+project_dir="$(CDPATH= cd -- "$(dirname -- "$0")/.." && pwd)"
+target=x86_64-pc-windows-msvc
+build_dir="$project_dir/target/windows"
+package_dir="$build_dir/flectar-mail"
+zip_path="$build_dir/flectar-mail-windows-x64.zip"
+
+# Debian and Ubuntu install clang-cl, lld-link, llvm-lib and llvm-rc only
+# under the versioned LLVM prefix.
+if ! command -v clang-cl >/dev/null 2>&1 || ! command -v llvm-rc >/dev/null 2>&1; then
+  llvm_bin="$(ls -d /usr/lib/llvm-*/bin 2>/dev/null | sort -V | tail -n 1 || true)"
+  if [[ -n "$llvm_bin" ]]; then
+    export PATH="$llvm_bin:$PATH"
+  fi
+fi
+for tool in cargo-xwin clang-cl lld-link llvm-lib llvm-rc; do
+  if ! command -v "$tool" >/dev/null 2>&1; then
+    printf '%s is missing. See the setup steps at the top of %s.\n' "$tool" "$0" >&2
+    exit 1
+  fi
+done
+
+# Match the release workflow: link the C runtime statically so Windows needs no
+# VC++ redistributable. RUSTFLAGS replaces .cargo/config.toml, so retain its
+# stack setting.
+export RUSTFLAGS="-C target-feature=+crt-static -C link-arg=/STACK:8000000"
+build_args=(--locked --release --target "$target" --bin flectar-mail --manifest-path "$project_dir/Cargo.toml" --no-default-features --features remote-content,gpu-renderer)
+if [[ -n "${FLECTAR_APP_FEATURES:-}" ]]; then
+  build_args+=(--features "$FLECTAR_APP_FEATURES")
+fi
+if [[ "${FLECTAR_SKIP_BUILD:-0}" != "1" ]]; then
+  # Extra arguments are passed to Cargo, for example -j 4.
+  cargo xwin build "${build_args[@]}" "$@"
+fi
+
+executable="$project_dir/target/$target/release/flectar-mail.exe"
+if [[ ! -f "$executable" ]]; then
+  printf 'The Windows executable was not built: %s\n' "$executable" >&2
+  exit 1
+fi
+
+rm -rf "$package_dir"
+mkdir -p "$package_dir/LICENSES/Google Sans Flex" "$package_dir/LICENSES/Noto Emoji"
+python3 "$project_dir/scripts/stage-pdfium.py" win-x64 "$package_dir" --cache "$build_dir/pdfium-cache"
+cp "$executable" "$project_dir"/{README.md,LICENSE,THIRD_PARTY_NOTICES.md} "$package_dir/"
+cp -R "$project_dir"/LICENSES/. "$package_dir/LICENSES/"
+cp "$project_dir"/resources/fonts/google-sans-flex/{OFL.txt,README.md} "$package_dir/LICENSES/Google Sans Flex/"
+cp "$project_dir"/resources/fonts/noto-emoji/{OFL.txt,README.md} "$package_dir/LICENSES/Noto Emoji/"
+
+rm -f "$zip_path"
+(cd "$package_dir" && python3 -m zipfile -c "$zip_path" *)
+printf 'Windows build ready:\n  %s\n  %s\n' "$package_dir/flectar-mail.exe" "$zip_path"