fix(setup): skip sudo apt when the Windows toolchain is already installed

setup.sh --win ran `sudo apt-get install` (and update-alternatives)
unconditionally, forcing the whole run under sudo even when mingw-w64 was
already present. Running setup as root makes the daemon cross-compile run as
root, leaving root-owned artifacts under external/dragonx that break `make
clean` on a later non-sudo build (stale objects relink -> mingw link failure
recurs). Gate the apt/update-alternatives block behind a presence check so
`./setup.sh --win` runs sudo-free (and the daemon build as the invoking user)
when the toolchain is already installed.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

setup.sh

@@ -284,6 +284,14 @@ fi
 header "Windows Cross-Compile"
 
 if $SETUP_WIN; then
+    # Only touch apt / update-alternatives (which need sudo) when the toolchain is missing. If it is
+    # already installed, skip them so `./setup.sh --win` can run WITHOUT sudo — important because the
+    # daemon cross-compile that follows should run as the invoking user. Running the whole setup under
+    # sudo leaves root-owned build artifacts under external/dragonx, which then break `make clean` on
+    # a later non-sudo build (stale objects get relinked -> the mingw link failure recurs).
+    if has_cmd x86_64-w64-mingw32-g++-posix || has_cmd x86_64-w64-mingw32-g++; then
+        ok "Windows cross-compile toolchain already present — skipping apt install"
+    else
         win_pkgs="$(get_pkgs win)"
         if [[ -n "$win_pkgs" ]]; then
             install_pkgs "$win_pkgs" "Windows cross-compile"
@@ -298,6 +306,7 @@ if $SETUP_WIN; then
                     /usr/bin/x86_64-w64-mingw32-g++-posix 2>/dev/null || true
             fi
         fi
+    fi
 
     # Fetch libsodium for Windows
     if [[ ! -f "$PROJECT_DIR/libs/libsodium-win/lib/libsodium.a" ]]; then