Replace free-text institution entry with a searchable bank picker

GET /api/banking/institutions lists the banks Enable Banking can connect
for a country (personal AIS, connectable consents only), with logos
restricted to https Enable Banking hosts to match the CSP image
allowlist. The connect form offers a filterable dropdown with bank
logos, falling back to the previous free-text input when the list is
unavailable or banking is not configured.

internal/app/sync_test.go

@@ -24,6 +24,9 @@ type bankScenario struct {
 func (b *bankScenario) Authorize(context.Context, string, string, string) (string, error) {
 	return "https://bank.example/authorize", nil
 }
+func (b *bankScenario) Institutions(context.Context, string) ([]banking.Institution, error) {
+	return []banking.Institution{{Name: "N26", Country: "DE"}}, nil
+}
 func (b *bankScenario) Exchange(context.Context, string) (banking.Session, error) {
 	return b.session, nil
 }