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.

stores/email-store.ts

@@ -2884,7 +2884,14 @@ export const useEmailStore = create<EmailStore>((set, get) => ({
       const merged: Email[] = [...refreshedEmails];
       const mergedIds = new Set(refreshedEmails.map((e: Email) => e.id));
       const insertedCount = Math.max((result.total || 0) - previousTotal, 0);
-      const appendFromIndex = Math.max(refreshedEmails.length - insertedCount, 0);
+      // Derive the cutoff from the page size, not from the refreshed list's
+      // length: when the folder shrank (a deletion - e.g. the draft of a just
+      // sent mail), the fresh page is shorter than the stale list and a
+      // length-based cutoff re-appends the deleted rows from stale local
+      // state. That ghost row is how "sent mail still shows as draft"
+      // reports happen (#592) - and re-sending the ghost delivers the mail
+      // again.
+      const appendFromIndex = Math.max(emailsPerPage - insertedCount, 0);
 
       for (const email of currentEmails.slice(appendFromIndex)) {
         if (!mergedIds.has(email.id)) {