feat: add extra-compact density option and fix font size scaling

- Add 'extra-compact' density level that hides avatars and preview
  text, showing only sender and subject for maximum information density
- Add visual density preview in appearance settings showing a 3-row
  email list mockup that reflects each density option
- Fix global font size setting only changing line height by moving
  font-size from body to :root so rem-based Tailwind classes scale
- Apply extra-compact behavior to email list items, thread list items,
  thread email items, and thread conversation view (EmailCard)
- Update email list virtualizer size estimates for extra-compact
- Add extra-compact translations for all 8 locales

locales/es/common.json

@@ -512,6 +512,7 @@
       "list_density": {
         "label": "Densidad",
         "description": "Controle el espaciado en toda la interfaz",
+        "extra_compact": "Extra compacto",
         "compact": "Compacto",
         "regular": "Regular",
         "comfortable": "Cómodo"