feat(panel): detect import encryption from file extension, not a checkbox

The import form used to ask users to tick "the file is encrypted" before
showing the password field, even though the server already decides purely
from the envelope magic bytes. Reveal the password field automatically for
a .spde file (hide it for .json) so the checkbox is no longer needed.

internal/web/static/panel.js

@@ -102,6 +102,37 @@
     });
   }
 
+  // --- Import password field shown based on the chosen file's extension ---
+  // The domain-import file decides for itself whether it is encrypted (the
+  // server checks the envelope magic, not a checkbox), so the panel offers
+  // the password field the same way: reveal it for a .spde file, hide and
+  // clear it for a plain .json one. An unrecognised name leaves the field
+  // visible rather than guessing wrong and hiding a password the file needs.
+  function syncImportPasswordField(input) {
+    var form = input.closest("form");
+    var fields = form && form.querySelector("[data-import-password-fields]");
+    if (!fields) {
+      return;
+    }
+    var name = (input.files && input.files[0] && input.files[0].name || "").toLowerCase();
+    var hide = name !== "" && /\.json$/.test(name);
+    fields.hidden = hide;
+    if (hide) {
+      fields.querySelectorAll("input").forEach(function (pw) {
+        pw.value = "";
+      });
+    }
+  }
+
+  function initImportPasswordField(root) {
+    root.querySelectorAll("input[data-import-file]").forEach(function (input) {
+      syncImportPasswordField(input);
+      input.addEventListener("change", function () {
+        syncImportPasswordField(input);
+      });
+    });
+  }
+
   // --- Section index follows the page -----------------------------------
   // The long pages list their own sections in the navigation column (the
   // "sections" template). Marking the one currently in view turns that list
@@ -167,6 +198,7 @@
   document.addEventListener("DOMContentLoaded", function () {
     initAddressFields(document);
     initEncryptFields(document);
+    initImportPasswordField(document);
     initSectionIndex();
   });