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.

web/src/api.ts

@@ -73,6 +73,11 @@ export interface Connection {
   valid_until: string;
   error: string;
 }
+export interface Institution {
+  name: string;
+  country: string;
+  logo?: string;
+}
 export interface State {
   data: Dataset;
   revision: string;