[Imapsync] Use dropdown for custom params

data/web/lang/lang.en-gb.json

@@ -560,6 +560,7 @@
         "imapsync_source_port_invalid": "Invalid port",
         "imapsync_source_enc_invalid": "Invalid encryption method",
         "imapsync_source_auth_invalid": "Invalid authentication method",
+        "imapsync_custom_param_invalid": "Custom parameters contain a disallowed imapsync option",
         "imapsync_source_oauth_required": "OAuth2 requires token endpoint, client ID, client secret and scope",
         "imapsync_source_oauth_endpoint_invalid": "Invalid OAuth2 endpoint URL",
         "imapsync_source_oauth_extra_invalid": "OAuth2 extra parameters must be valid JSON",
@@ -1297,7 +1298,10 @@
         "timeout2": "Timeout for connection to local host",
         "maxbytespersecond": "Max. bytes per second <br><small>(0 = unlimited)</small>",
         "custom_params": "Custom parameters",
-        "custom_params_hint": "Right: --param=xy, wrong: --param xy",
+        "custom_params_hint": "Add one imapsync option per row. Only allowlisted options are accepted; the value may contain any character (spaces, regex, …) and is passed as a single argument.",
+        "custom_param_option": "Option",
+        "custom_param_value": "Value (optional)",
+        "custom_param_add": "Add parameter",
         "order_id": "Position",
         "order_id_hint": "Lower position runs first; setting it re-sorts the global queue.",
         "set_position": "Set position"