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

locales/de/common.json

@@ -510,8 +510,8 @@
         "large": "Groß"
       },
       "list_density": {
-        "label": "Listendichte",
-        "description": "Abstand in E-Mail-Listen steuern",
+        "label": "Dichte",
+        "description": "Abstände in der Benutzeroberfläche steuern",
         "compact": "Kompakt",
         "regular": "Normal",
         "comfortable": "Komfortabel"