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>

backend/tests/outbox/test_outbox.py

@@ -398,6 +398,8 @@ async def test_send_creates_outbox_and_defers(ctx: Ctx) -> None:
     resp = await ctx.client.post("/api/send", json={"draft_id": did}, headers=_hdr())
     assert resp.status_code == 200
     outbox_id = resp.json()["outbox_id"]
+    # The window as a duration too, so the browser's clock does not matter.
+    assert 0 < resp.json()["undo_seconds"] <= get_settings().undo_seconds
 
     ob = await ctx.session.get(Outbox, uuid.UUID(outbox_id))
     assert ob is not None and ob.state == "undo_window"