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

app/globals.css

@@ -23,6 +23,13 @@
   --font-size-base: 16px;
   --list-item-height: 48px;
   --transition-duration: 0.2s;
+
+  /* Density spacing variables (defaults = regular) */
+  --density-item-py: 12px;
+  --density-item-gap: 12px;
+  --density-header-py: 12px;
+  --density-card-p: 16px;
+  --density-sidebar-py: 4px;
 }
 
 .dark {