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/controllers/settings/providers_controller_test.rb

@@ -8,6 +8,12 @@ class Settings::ProvidersControllerTest < ActionDispatch::IntegrationTest
     Provider::Factory.ensure_adapters_loaded
   end
 
+  test "GET /settings/bank_sync redirects permanently to /settings/providers" do
+    get "/settings/bank_sync"
+    assert_redirected_to "/settings/providers"
+    assert_equal 301, response.status
+  end
+
   test "can access when self hosting is disabled (managed mode)" do
     Rails.configuration.stubs(:app_mode).returns("managed".inquiry)
     get settings_providers_url