fix: make density setting functional across entire UI

Rename "List Density" to "Density" and apply spacing changes globally
instead of only to email list item heights.

- Replace hardcoded padding/gap values with CSS custom properties
  (--density-item-py, --density-item-gap, --density-header-py,
  --density-card-p, --density-sidebar-py) set via JS on :root
- Apply density-responsive spacing to email list items, thread views,
  email viewer, sidebar, navigation rail, contacts, and calendar
- Use inline styles instead of Tailwind arbitrary value classes to
  avoid Turbopack compilation hangs
- Rename listDensity → density in store, types, and components
- Add persist migration (v1 → v2) and onRehydrateStorage callback
- Update all 8 locale files with broadened labels/descriptions

components/contacts/contact-list-item.tsx

@@ -22,10 +22,11 @@ export function ContactListItem({ contact, isSelected, onClick }: ContactListIte
     <button
       onClick={onClick}
       className={cn(
-        "w-full flex items-center gap-3 px-4 py-3 text-left transition-colors",
+        "w-full flex items-center px-4 text-left transition-colors",
         "hover:bg-muted",
         isSelected && "bg-accent text-accent-foreground"
       )}
+      style={{ gap: 'var(--density-item-gap)', paddingBlock: 'var(--density-item-py)' }}
     >
       <Avatar name={name} email={email} size="sm" />
       <div className="flex-1 min-w-0">