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"

locales/en/common.json

@@ -510,8 +510,8 @@
         "large": "Large"
       },
       "list_density": {
-        "label": "List Density",
-        "description": "Control spacing in email lists",
+        "label": "Density",
+        "description": "Control spacing and padding across the UI",
         "compact": "Compact",
         "regular": "Regular",
         "comfortable": "Comfortable"

locales/es/common.json

@@ -510,8 +510,8 @@
         "large": "Grande"
       },
       "list_density": {
-        "label": "Densidad de Lista",
-        "description": "Controle el espaciado en las listas de correo",
+        "label": "Densidad",
+        "description": "Controle el espaciado en toda la interfaz",
         "compact": "Compacto",
         "regular": "Regular",
         "comfortable": "Cómodo"

locales/fr/common.json

@@ -510,8 +510,8 @@
         "large": "Grande"
       },
       "list_density": {
-        "label": "Densité de la liste",
-        "description": "Contrôlez l'espacement dans les listes d'emails",
+        "label": "Densité",
+        "description": "Contrôlez l'espacement dans toute l'interface",
         "compact": "Compacte",
         "regular": "Normale",
         "comfortable": "Confortable"

locales/it/common.json

@@ -510,8 +510,8 @@
         "large": "Grande"
       },
       "list_density": {
-        "label": "Densità elenco",
-        "description": "Controlla la spaziatura negli elenchi email",
+        "label": "Densità",
+        "description": "Controlla la spaziatura in tutta l'interfaccia",
         "compact": "Compatto",
         "regular": "Normale",
         "comfortable": "Comodo"

locales/ja/common.json

@@ -510,8 +510,8 @@
         "large": "大"
       },
       "list_density": {
-        "label": "リストの密度",
-        "description": "メールリストの間隔を調整",
+        "label": "密度",
+        "description": "UI全体の間隔を調整",
         "compact": "コンパクト",
         "regular": "標準",
         "comfortable": "ゆったり"

locales/nl/common.json

@@ -510,8 +510,8 @@
         "large": "Groot"
       },
       "list_density": {
-        "label": "Lijstdichtheid",
-        "description": "Regeleer de ruimte in e-maillijsten",
+        "label": "Dichtheid",
+        "description": "Regeleer de ruimte in de gehele interface",
         "compact": "Compact",
         "regular": "Normaal",
         "comfortable": "Comfortabel"

locales/pt/common.json

@@ -510,8 +510,8 @@
         "large": "Grande"
       },
       "list_density": {
-        "label": "Densidade da Lista",
-        "description": "Controle o espaçamento nas listas de e-mail",
+        "label": "Densidade",
+        "description": "Controle o espaçamento em toda a interface",
         "compact": "Compacto",
         "regular": "Regular",
         "comfortable": "Confortável"