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/MailLayout.tsx

@@ -583,8 +583,7 @@ export function MailLayout() {
                     color={a.color}
                     syncing={
                       a.state === "backfilling" &&
-                      a.backfill_total > 0 &&
-                      a.backfill_done < a.backfill_total
+                      (a.backfill_total === 0 || a.backfill_done < a.backfill_total)
                     }
                     total={a.backfill_total}
                     done={a.backfill_done}