fix(i18n): pluralize units (1 lata vs 3 latas)

[Endika]

Summary

Wires i18next's built-in plural support so unit labels render the correct singular/plural form based on quantity. Previously the UI always showed plural ("1 latas"), now it shows "1 lata" / "3 latas" correctly.

• displayUnit(unit, t, count?) now takes an optional count and passes it to t() for plural lookup. Without count it returns the base plural (used for dropdown labels).
• Updated 5 call sites to pass quantity: formatShoppingListText, ConsumptionSummary, ExpenseForm, and 2 spots in PurchasesTab.
• All 6 locales (ca / en / es / eu / gl / va) gained _one and _other keys for: units, bottles, cans, liters, grams, bag, tray. kg and single stay invariant. Base plural keys preserved for backward-compatible dropdown rendering.

Test plan

• Open share modal with an item of 1 botella and another of 3 botellas → singular/plural in each line
• "Lo que te toca" detail list with mixed quantities → same
• Switch locale → plurals correct in each language