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/components/thread/ThreadView.tsx

@@ -343,7 +343,7 @@ function MessageItem({
   const [showDetails, setShowDetails] = useState(false);
   const replyBtn = (kind: string, label: string, icon: string) => (
     <button
-      className="wm-online-only flex items-center gap-1.5 rounded-md border border-app-border px-3 py-1.5 text-xs text-app-fg hover:bg-app-hover"
+      className="flex items-center gap-1.5 rounded-md border border-app-border px-3 py-1.5 text-xs text-app-fg hover:bg-app-hover"
       onClick={() => onReply(kind)}
     >
       <Icon name={icon} size={14} />
@@ -442,7 +442,7 @@ function MessageItem({
                   </button>
                   {canReply && (
                     <button
-                      className="wm-online-only flex items-center gap-1 text-app-muted hover:text-app-fg"
+                      className="flex items-center gap-1 text-app-muted hover:text-app-fg"
                       onClick={() => onReply(multiRecipient ? "reply_all" : "reply")}
                     >
                       <Icon name={multiRecipient ? "reply-all" : "reply"} size={14} />