chore: sync install.sh to gh-pages, update install URL

[algirdasc]

Changes

GitHub Actions workflow

.github/workflows/sync-gh-pages.yml — triggers on push to main when install.sh changes (also workflow_dispatch for manual runs):

1. Checkout gh-pages
2. Copy install.sh from origin/main
3. Commit + push if changed ([skip ci] to avoid loop)

URL update

README.md install command updated:

# Before
curl -fsSL https://raw.githubusercontent.com/algirdasc/expensave/main/install.sh | sudo bash

# After
curl -fsSL https://expensave.app/install.sh | sudo bash

Landing page URL will be updated separately in gh-pages branch.