## What
Reworks the two top summary cards on the **Cashflow** page so they no
longer both show net flow.
### Net Cashflow (card 1)
- Shows the net amount **and** its share of income (%).
- Compares **both** the amount and the percentage with the previous
period.
### Saved & Invested (card 2, replaces "Savings Rate")
- Headline = money moved to savings + investment categories (amount).
- Percentage = that total as a share of **net cashflow** (income −
expenses).
- Compares amount and percentage vs. the previous period.
- Keeps the per-category Saved / Invested breakdown at the bottom.
- Adds a **click-triggered help popover** (shadcn `Popover`, works on
touch + desktop, dismisses on outside click / Escape) explaining the
numbers come from transactions using a **"saving"** or **"investment"**
category type.
### i18n
- The card's "Saved" label uses a dedicated key so it renders
**"Ahorrado"** on the cashflow card while form buttons keep
**"Guardado"**. A minimal `lang/en.json` keeps the English label as
"Saved".
- Spanish strings added for the new card + popover copy.
## Testing
- `vitest` — net-cashflow + saved-invested card specs (amount/percentage
comparisons, click-to-reveal popover)
- Full `pest` suite (excl. Browser) — green; localization key-coverage
test passes
- `pint --test`, `prettier --check`, `eslint` — clean
- production `bun run build` — succeeds
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