Adds a segmented Monthly/YTD toggle next to the reference-month picker that
flips the four KPI cards (income, expenses, net, savings rate) between the
reference-month value (unchanged default) and a Year-to-Date cumulative view.
In YTD mode, the "current" value sums January to the reference month of the
reference year; MoM delta compares it to Jan to (refMonth - 1) of the same
year (null in January, since no prior YTD window exists); YoY delta compares
it to Jan to refMonth of the previous year; savings rate is recomputed from
YTD income and expenses, and stays null when YTD income is zero.
The 13-month sparkline, top movers, seasonality and budget adherence cards
remain monthly regardless of the toggle (by design). The savings-rate tooltip
is now dynamic and mirrors the active mode. The mode is persisted in
localStorage under `reports-cartes-period-mode`.
Also adds a dedicated Cartes section to `docs/guide-utilisateur.md` covering
the four KPI formulas, the Monthly/YTD toggle and its effect on deltas, the
sparkline, top movers, seasonality, budget adherence and the savings-rate
edge case. Mirrored in the in-app `docs.reports` i18n tree (features/steps/
tips extended) for both FR and EN.
No SQL migration: YTD sums are computed from the already-fetched
`flowByMonth` map, so no extra round trip.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
3be05db41a