feat(settings): retire /settings/bank_sync; merge into providers page

- Delete Settings::BankSyncController and its views (the providers page is
  now a strict superset of what bank_sync offered)
- Add permanent 301 redirect: GET /settings/bank_sync → /settings/providers
- Collapse nav to a single "Bank Sync" entry pointing at /settings/providers;
  remove the duplicate admin-only "Providers" entry from the Advanced section
- Remove "Providers" from SETTINGS_ORDER; point "Bank Sync" at
  settings_providers_path for next/prev navigation
- Rename page title to "Bank Sync"; replace admin-credential lede with
  user-facing copy ("Connect external accounts…")
- Update breadcrumb: Home → Bank sync
- Add controller test asserting 301 status and Location header

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/system/settings_test.rb

@@ -7,7 +7,7 @@ class SettingsTest < ApplicationSystemTestCase
     # Base settings available to all users
     @settings_links = [
       [ "Accounts", accounts_path ],
-      [ "Bank Sync", settings_bank_sync_path ],
+      [ "Bank Sync", settings_providers_path ],
       [ "Preferences", settings_preferences_path ],
       [ "Profile Info", settings_profile_path ],
       [ "Security", settings_security_path ],