Offline compose: write mail offline, Scheduled folder, undo countdown fix

Compose, Reply and Forward work offline. Drafts are kept on the device
(IndexedDB, lib/scheduled.ts), with files attached offline; Send becomes
"Schedule" and puts the message in a new Scheduled folder. When the server
is reachable again, scheduled messages are sent through the normal outbox
(Undo offered for a single one) and offline drafts become server drafts —
files uploaded first, each step recorded so nothing is created or sent
twice, under a Web Lock so two tabs never send the same message. Drafts
written offline show in Drafts as "On this device". Sign-out sends what
is scheduled, then wipes the device's copy.

Also: offline failures from keyboard shortcuts show "Not available
offline"; no recipient suggestions are asked for offline; a reply no
longer quotes an error page when the original is not saved.

The undo countdown counted to the server's deadline on the browser's
clock, so a few seconds of skew showed (18 s instead of 10). /send now
also returns undo_seconds, and the composer counts down from that.

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

frontend/src/i18n/en.ts

@@ -556,6 +556,19 @@ export const en = {
   "offline.notAvailable": "Not available offline",
   "offline.threadNotSaved": "This conversation isn't saved on this device.",
   "offline.folderNotSaved": "This folder isn't saved on this device.",
+  "offline.noticeScheduled": "Saved to Scheduled — it will be sent when you're back online.",
+  "offline.noticeSendingScheduled": "Sending {count} scheduled messages…",
+  "offline.draftsNotSaved": "Only drafts written on this device are available offline.",
+  "compose.sendLater": "Schedule",
+  "compose.savedOnDevice": "Saved on this device",
+  "compose.attachOnDevice": "On this device — uploaded when you're back online",
+  "compose.onDevice": "On this device",
+  "nav.scheduled": "Scheduled",
+  "scheduled.help": "Sent automatically when you're back online.",
+  "scheduled.empty": "Nothing scheduled.",
+  "scheduled.waiting": "Waiting",
+  "scheduled.sending": "Sending…",
+  "scheduled.failed": "Not sent",
   "offline.searchPlaceholder": "Search (offline)",
   "offline.cantReach": "Can't reach the server.",
   "offline.settingTitle": "Offline access",