Phone: swipe right to delete, Mailbox display with the dividing lines

On a phone, a conversation dragged to the right shows a red bin behind it;
let go past a third of the row and it goes to Trash, short of that it
springs back. Vertical drags still scroll (touch-action: pan-y), and a swipe
never opens the conversation. "Moved to Trash · Undo" shows for 5 s and the
move is made only after that, like Undo send — so Undo just shows the row
again; leaving the page still deletes. Not in Trash (it would be for good)
and not offline.

The ⋯ menu on a phone gains Mailbox display, holding only "Show a dividing
line between rows" (a phone has no layouts to choose).

The corner toast takes an optional action (label + run) and duration.

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

frontend/src/i18n/en.ts

@@ -184,6 +184,7 @@ export const en = {
   "panel.noAttachments": "No attachments",
   "panel.download": "Download",
   "mail.emptyConfirm": "Permanently delete everything in this folder? This can't be undone.",
+  "mail.movedToTrash": "Moved to Trash",
   "mail.loadFailed": "Couldn't load this message.",
   "mail.imagesBlocked": "Images from this sender are blocked.",
   "mail.loadImages": "Load images",