Adds a second historical line to the existing cash-balance chart showing
what the balance would be if all outstanding player dues were paid,
reconstructed per month the same way the existing balance line is.
Brainstormed with the user: three charts on the existing overview page
(balance history, monthly income/expense, top-10 outstanding players),
Chart.js as dependency-free charting lib, new backend aggregation
endpoint since none of the existing endpoints group transactions by
time or category.
.env already exists locally with valid creds (the deleted repo-root
.env was an unused duplicate), and no MailDev infra exists for the
e2e mail tests, so the plan shifts to a MailService unit test plus
one manual send instead of touching .env or standing up e2e infra.
Captures the brainstorming outcome for fixing the broken mail dispatch,
removing the mail-only nestjs-i18n setup in favor of hardcoded German
copy, and redesigning the registration/forgot-password email templates.
