Fix: stop resurrecting deleted rows in the mailbox refresh merge

Fixes #592.

refreshCurrentMailbox merges the refreshed first page with the already
loaded list, appending existing entries beyond a cutoff. That cutoff
was derived from the refreshed list's length - so whenever a folder
shrank, the fresh page was shorter than the stale list and the loop
re-appended the deleted rows from stale local state, despite the
comment right above promising the opposite.

The visible result is the reported bug: after sending a draft, the
Drafts view keeps showing a ghost row for the already-destroyed draft.
The send actually succeeded - resending the ghost delivers the mail
again, which we reproduced with a live JMAP trace: four successful
submissions, an empty server-side Drafts folder, a notFound ghost id,
and five delivered copies. Deriving the cutoff from the page size
fixes the shrink case while preserving the merge's intent for
arrivals and loaded deeper pages; regression tests cover all three
shapes.

Also surface post-send filing failures instead of dropping them, as
flagged in 4dc76bbb's follow-up note: a rejected onSuccessUpdateEmail
patch or old-draft destroy now logs the server's error details and
returns a filingError on SendEmailResult, and the UI shows a warning
toast (all 23 locales) so a stale draft row is never again mistaken
for a failed send. A plugin veto of the send leaves a debug trace.

app/(main)/[locale]/page.tsx

@@ -1222,6 +1222,14 @@ export default function Home() {
 
       const result = await sendEmail(client, data.to, data.subject, data.body, data.cc, data.bcc, data.identityId, data.fromEmail, data.draftId, data.fromName, data.htmlBody, data.attachments, data.inReplyTo, data.references, data.delayedUntil, data.envelopeMailFrom, { requestReadReceipt: data.requestReadReceipt });
       setShowComposer(false);
+      if (result.filingError) {
+        // The mail went out, but a post-send step (filing to Sent /
+        // removing the old draft) was rejected - warn instead of staying
+        // silent, so a stale draft row is not mistaken for a failed send
+        // and re-sent (#592).
+        const toastInstance = (await import('sonner')).toast;
+        toastInstance.warning(t('email_composer.send_filing_warning'));
+      }
       if (result.scheduled) {
         await refreshScheduledMetadata(client);
         if (isScheduledView) await fetchScheduledEmails(client);