Reading pane layouts: Side, Bottom (under the list) and Off (single panel)

Choose in Settings > Appearance, or the list's ⋯ menu > Mailbox display (a
popover with three preview cards; picking one applies it and closes it).
Stored per device, like the theme; phones keep their own single panel.

- Side: the existing three columns.
- Bottom: the list above, the reading pane below, 45/55 until the 3px divider
  is dragged (height remembered). The pane's top bar takes the list filter
  bar's colour; the empty-pane picture shrinks to fit.
- Off: one panel. The list, until a conversation opens in its place with a
  Back button; bulk actions move to a bar above the list. This reuses the
  phone's list/pane switching with desktop styling.
- In Bottom and Off the side-panel toggle sits beside the search box (Off also
  keeps it in the conversation toolbar, since the list is hidden there).
- An expanded composer covers the reading pane in every arrangement: the lower
  pane under the list, the whole panel with the pane off.

Resizer gains a horizontal orientation; layoutDock takes the pane height.

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

frontend/src/i18n/en.ts

@@ -462,6 +462,12 @@ export const en = {
   "admin.saving": "Saving…",
   "admin.saved": "Saved",
   "admin.quotaInvalid": "Enter a size greater than 0.",
+  "display.title": "Mailbox display",
+  "display.readingPane": "Reading pane",
+  "display.side": "Side",
+  "display.bottom": "Bottom",
+  "display.off": "Off",
+  "display.phoneNote": "On a phone, mail always opens in a single panel.",
   "admin.mail": "Outgoing system mail",
   "admin.mailHelp":
     "The mail account the server sends password-reset links from. Without it, “Forgot password?” sends nothing.",