New nullable columns on Battery: size, chemistry, capacity_mah,
tested_capacity_mah, tested_date, charge_cycles, purchase_date.
Battery detail page shows all populated fields and a full edit form
with select dropdowns for size and chemistry (with Other fallback).
Capacity health % shown in green/orange/red when both nominal and
tested capacity are set. Dashboard gains a Size column.
datalist only shows suggestions when typing, not on click. Switched to
a <select> showing all known brands plus a 'New brand...' option that
reveals a text input for entering a new brand name.
iterminate
