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/tailwind.config.js

@@ -4,6 +4,10 @@ export default {
   content: ["./index.html", "./admin.html", "./src/**/*.{ts,tsx}"],
   theme: {
     extend: {
+      // The interface-size setting zooms <html>, and zoom scales viewport units
+      // too: divide them back so a full-height layout still fits (lib/uiScale.ts).
+      height: { screen: "calc(100vh / var(--ui-zoom, 1))" },
+      minHeight: { screen: "calc(100vh / var(--ui-zoom, 1))" },
       fontFamily: {
         sans: ['"Lato"', "system-ui", "-apple-system", '"Segoe UI"', "Roboto", "sans-serif"],
       },