Sync works on the deployed site; add reproducible build and deploy script

The Pages deployment is now the primary copy, so two things had to change:

- Hosted pages fall back to steam-sync.py on 127.0.0.1:8765 instead of
  refusing to sync. steam-sync.py now answers Chrome's Private Network
  Access preflight (Access-Control-Allow-Private-Network), which is what a
  public https page needs before it may call into localhost.
- The build sources lived in /tmp and would have been lost. They now live in
  build/, and build.py outputs to the repo root. deploy.sh rebuilds,
  commits and pushes in one step.

Editing index.html directly is no longer the workflow - edit build/ and run
./deploy.sh.

deploy.sh

@@ -0,0 +1,31 @@
+#!/usr/bin/env bash
+#
+# Rebuild the companion and publish it to the Pages branch.
+#
+#   ./deploy.sh                 commit as "Update companion"
+#   ./deploy.sh "fix dog tags"  custom commit message
+#
+# The Pages site rebuilds itself from the pushed branch, usually within ~15s:
+#   https://rain.pages.melonbread.xyz/la-li-lu-le-lo/
+#
+set -euo pipefail
+cd "$(dirname "$0")"
+
+echo "==> building"
+python3 build/build.py
+
+if git diff --quiet && git diff --cached --quiet; then
+  echo "==> nothing changed, nothing to publish"
+  exit 0
+fi
+
+echo "==> committing"
+git add -A
+git commit -m "${1:-Update companion}"
+
+echo "==> pushing"
+git push origin pages
+
+echo
+echo "done. live at https://rain.pages.melonbread.xyz/la-li-lu-le-lo/"
+echo "(Pages usually picks it up within ~15 seconds)"