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>

app/controllers/settings/bank_sync_controller.rb

@@ -1,43 +0,0 @@
-class Settings::BankSyncController < ApplicationController
-  layout "settings"
-
-  def show
-    @providers = [
-      {
-        name: "Lunch Flow",
-        description: "US, Canada, UK, EU, Brazil and Asia through multiple open banking providers.",
-        path: "https://lunchflow.app/features/sure-integration?atp=BiDIYS",
-        target: "_blank",
-        rel: "noopener noreferrer"
-      },
-      {
-        name: "Plaid",
-        description: "US & Canada bank connections with transactions, investments, and liabilities.",
-        path: "https://github.com/we-promise/sure/blob/main/docs/hosting/plaid.md",
-        target: "_blank",
-        rel: "noopener noreferrer"
-      },
-      {
-        name: "SimpleFIN",
-        description: "US & Canada connections via SimpleFIN protocol.",
-        path: "https://beta-bridge.simplefin.org",
-        target: "_blank",
-        rel: "noopener noreferrer"
-      },
-      {
-        name: "Enable Banking (beta)",
-        description: "European bank connections via open banking APIs across multiple countries.",
-        path: "https://enablebanking.com",
-        target: "_blank",
-        rel: "noopener noreferrer"
-      },
-      {
-        name: "Sophtron (alpha)",
-        description: "US & Canada bank, credit card, investment, loan, insurance, utility, and other connections.",
-        path: "https://www.sophtron.com/",
-        target: "_blank",
-        rel: "noopener noreferrer"
-      }
-    ]
-  end
-end