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

app/globals.css

@@ -3,6 +3,8 @@
 @custom-variant dark (&:where(.dark, .dark *));
 
 :root {
+  font-size: var(--font-size-base);
+
   --color-border: #e2e8f0;
   --color-input: #e2e8f0;
   --color-ring: #94a3b8;
@@ -79,7 +81,6 @@ body {
     system-ui,
     -apple-system,
     sans-serif;
-  font-size: var(--font-size-base);
   font-feature-settings:
     "rlig" 1,
     "calt" 1;