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/consent_test.go

@@ -23,6 +23,10 @@ func (b *historyBank) Authorize(_ context.Context, _, _, state string) (string,
 	return "https://bank.example/authorize", nil
 }
 
+func (b *historyBank) Institutions(context.Context, string) ([]banking.Institution, error) {
+	return []banking.Institution{{Name: "N26", Country: "DE"}}, nil
+}
+
 func (b *historyBank) Transactions(_ context.Context, account domain.Account, from, to string, _ bool) ([]domain.Facts, error) {
 	b.fromDates = append(b.fromDates, from)
 	var rows []domain.Facts