Add Wine-based runner image for Windows builds

- runner/Dockerfile: Debian + Wine + Windows CPython 3.12 + PyInstaller + act_runner
- runner/entrypoint.sh: self-register with the windows:host label
- workflow windows job now builds via winpython under Wine

runner/entrypoint.sh

@@ -0,0 +1,14 @@
+#!/bin/sh
+set -e
+
+# Register on first run, then run the daemon (host mode).
+if [ ! -f /data/.runner ]; then
+    echo "Registering Wine runner with Gitea…"
+    act_runner register --no-interactive \
+        --instance "${GITEA_INSTANCE_URL:?GITEA_INSTANCE_URL is required}" \
+        --token "${GITEA_RUNNER_REGISTRATION_TOKEN:?GITEA_RUNNER_REGISTRATION_TOKEN is required}" \
+        --name "${RUNNER_NAME:-windows-builder}" \
+        --labels "${RUNNER_LABELS:-windows:host}"
+fi
+
+exec act_runner daemon