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/app/api/compose.py

@@ -3,7 +3,7 @@
 from __future__ import annotations
 
 import uuid
-from datetime import timedelta
+from datetime import UTC, datetime, timedelta
 from typing import Annotated
 
 from fastapi import APIRouter, Depends, File, Query, UploadFile
@@ -300,7 +300,10 @@ async def send(body: SendRequest, user: CurrentUser, db: DbSession) -> SendRespo
         str(outbox.id),
         _defer_by=timedelta(seconds=get_settings().undo_seconds),
     )
-    return SendResponse(outbox_id=outbox.id, undo_until=outbox.send_after.isoformat())
+    left = max(0.0, (outbox.send_after - datetime.now(UTC)).total_seconds())
+    return SendResponse(
+        outbox_id=outbox.id, undo_until=outbox.send_after.isoformat(), undo_seconds=left
+    )
 
 
 @router.post("/send/{outbox_id}/undo")

backend/app/outbox/schemas.py

@@ -56,6 +56,9 @@ class SendRequest(BaseModel):
 class SendResponse(BaseModel):
     outbox_id: uuid.UUID
     undo_until: str
+    # The same deadline as seconds from now: the browser counts down from this,
+    # since its clock may be seconds off the server's.
+    undo_seconds: float
 
 
 class OutboxStateOut(BaseModel):

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"