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/i18n/en.ts

@@ -236,7 +236,15 @@ export const en = {
   "wizard.port": "Port",
   "wizard.security": "Security",
   "wizard.accountPassword": "Account password",
-  "wizard.appPasswordHint": "For Gmail/Yahoo/iCloud use an app password.",
+  "wizard.appPasswordHint": "For Gmail, Yahoo, iCloud, or Outlook use an app password.",
+  "wizard.help": "Help",
+  "wizard.appPasswordHelp.title": "Set up an app password",
+  "wizard.appPasswordHelp.intro":
+    "These providers need a one-time app password instead of your normal password:",
+  "wizard.appPasswordHelp.prefix": "Make sure to",
+  "wizard.appPasswordHelp.twoStep": "turn on 2-step verification",
+  "wizard.appPasswordHelp.middle": "if you haven't done it yet, and then",
+  "wizard.appPasswordHelp.generate": "generate the app password",
   "wizard.test": "Test & continue",
   "wizard.testing": "Testing connection…",
   "wizard.testOk": "Connection works.",
@@ -303,6 +311,7 @@ export const en = {
   "settings.downloadAllMail": "Sync remaining mail",
   "settings.syncedPartially": "Synced partially",
   "backfill.importing": "Importing messages… {done} of {total}",
+  "backfill.preparing": "Preparing to import messages…",
   "settings.appearance": "Appearance",
   "settings.handle": "Handle",
   "settings.recoveryEmail": "Recovery email",
@@ -376,6 +385,7 @@ export const en = {
   "settings.addAccount": "Add account",
   "settings.syncState": "State: {state}",
   "settings.confirmRemoveAccount": "Remove this account and all its mail?",
+  "settings.removingAccount": "Removing account…",
   "settings.editAccount": "Edit account",
   "settings.color": "Color",
   "settings.serverSettings": "Change server settings / password",