Multi-currency support. An account and a budget each carry a currency now:
creating either requires one, existing data is backfilled to XPF, and the
American Express importer splits into two paths. Read readme-admin.md before
upgrading across this tag.

✨ Hold accounts and budgets in several currencies
♻️ Turn the importers into a registry, split by currency
✨ Show and pick currencies across the app
✨ Name the currency only when the transaction page mixes them
✨ Warn that a bank can break its own importer
🔥 Drop the padding formatter, keep a currency's decimals
🐛 Keep data editable when its currency leaves circulation
🐛 Format cumulative summaries with the budget's decimals
📝 Document multi-currency accounts, budgets and importers
📝 Tell users what to click, not what the file looks like
✨ Let users sign up, change their password and delete their account
🔒️ Enforce a real password policy with a live checklist
💄 Fill the delete buttons with the themed error colour
🐛 Stop the delete dialog's warning icon from being clipped
🐛 Proxy the new account paths in ng serve
✅ Overhaul test suite: phase split, coverage, and mechanism-based taxonomy
🔒 Cover the security testing requirements across all endpoints
👷 Report integration coverage in CI