Backfill start-window indicator, account-delete spinner, app-password Help popup

- Backfill: show the sidebar spinner + "Preparing…" banner during the brief window
  after account creation when state=backfilling but total is not yet counted
  (total===0), so a freshly-added account shows progress immediately instead of
  looking done until a refresh. The stuck-at-100% guard is preserved.
- Settings: a centered spinner overlay ("Removing account…") covers the screen from
  confirm until the (slow) server cascade + accounts refetch complete, so the row
  no longer appears frozen before it disappears.
- Add-account wizard: the manual password hint now includes Outlook and gains a
  "Help" link opening a popup with per-provider direct links to enable 2-step
  verification and generate the app password (Gmail, Outlook, Yahoo, iCloud).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U72FZiF1W1pb8XxyWBTREA

frontend/src/routes/settings/Settings.tsx

@@ -251,9 +251,15 @@ function AccountsSection() {
   const [editing, setEditing] = useState<{ id: string; mode: "account" | "signature" } | null>(
     null,
   );
+  // Which account is being deleted — drives the blocking spinner overlay until the
+  // (slow) server cascade finishes and the refetched list no longer includes it.
+  const [deletingId, setDeletingId] = useState<string | null>(null);
   const remove = useMutation({
     mutationFn: (id: string) => api.deleteAccount(id),
-    onSuccess: () => queryClient.invalidateQueries({ queryKey: ["accounts"] }),
+    onSettled: async () => {
+      await queryClient.invalidateQueries({ queryKey: ["accounts"] });
+      setDeletingId(null);
+    },
   });
   const downloadAll = useMutation({
     mutationFn: (id: string) => api.updateAccount(id, { sync_window: "all" }),
@@ -325,7 +331,10 @@ function AccountsSection() {
                   <button
                     className="text-xs text-red-500"
                     onClick={() => {
-                      if (confirm(t("settings.confirmRemoveAccount"))) remove.mutate(a.id);
+                      if (confirm(t("settings.confirmRemoveAccount"))) {
+                        setDeletingId(a.id);
+                        remove.mutate(a.id);
+                      }
                     }}
                   >
                     {t("common.remove")}
@@ -336,6 +345,18 @@ function AccountsSection() {
           </li>
         ))}
       </ul>
+      {deletingId && (
+        <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/40 backdrop-blur-sm">
+          <div className="flex flex-col items-center gap-3 rounded-xl bg-app-elevated px-8 py-6 shadow-2xl">
+            <span
+              className="h-8 w-8 animate-spin rounded-full border-2 border-app-border border-t-app-accent"
+              role="status"
+              aria-label={t("settings.removingAccount")}
+            />
+            <span className="text-sm text-app-muted">{t("settings.removingAccount")}</span>
+          </div>
+        </div>
+      )}
     </section>
   );
 }