Interface size setting: Default, +10%, +20%

Mailbox display and Settings → Appearance get an interface-size picker,
stored per device in localStorage and applied as CSS zoom on <html>.
Viewport-height layouts divide by --ui-zoom, and code that turns mouse
coordinates or bounding rects into CSS lengths (resizers, compose dock,
emoji picker, editor popups and image handles, swipe-to-delete) divides
by the zoom factor.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

frontend/src/i18n/en.ts

@@ -670,6 +670,8 @@ export const en = {
   "display.bottom": "Bottom",
   "display.off": "Off",
   "display.dividers": "Show a dividing line between rows",
+  "display.size": "Interface size",
+  "display.sizeDefault": "Default",
   "admin.mail": "Outgoing system mail",
   "admin.mailHelp":
     "The mail account the server sends password-reset links from. Without it, “Forgot password?” sends nothing.",