Fix login modal nsec error visibility

- Removed `required` attribute from nsec secret input in `components/login-modal.html` to prevent browser native validation from suppressing custom error UI.
- Updated `js/ui/loginModalController.js` to ensure `required` property is disabled in JavaScript logic.
- Added comprehensive unit tests in `tests/login-modal-controller.test.mjs` to verify:
    - Custom error message is shown on empty submission.
    - Required attributes remain disabled when toggling between unlock/import modes.
    - NIP-46 handshake panel is correctly displayed (regression check).
- Verified fix with existing E2E test `tests/e2e/ui-modals.spec.ts` (NSEC error assertion passes).

js/ui/loginModalController.js

@@ -937,14 +937,14 @@ export default class LoginModalController {
       passphraseFields.hidden = !shouldShow;
       if (passphraseInput instanceof HTMLInputElement) {
         passphraseInput.disabled = !shouldShow;
-        passphraseInput.required = shouldShow;
+        passphraseInput.required = false;
         if (!shouldShow) {
           passphraseInput.value = "";
         }
       }
       if (passphraseConfirm instanceof HTMLInputElement) {
         passphraseConfirm.disabled = !shouldShow;
-        passphraseConfirm.required = shouldShow;
+        passphraseConfirm.required = false;
         if (!shouldShow) {
           passphraseConfirm.value = "";
         }
@@ -959,14 +959,14 @@ export default class LoginModalController {
       }
       if (unlockPassphrase instanceof HTMLInputElement) {
         unlockPassphrase.disabled = !usingUnlock;
-        unlockPassphrase.required = usingUnlock;
+        unlockPassphrase.required = false;
         if (!usingUnlock) {
           unlockPassphrase.value = "";
         }
       }
       if (secretField instanceof HTMLTextAreaElement) {
         secretField.disabled = usingUnlock;
-        secretField.required = !usingUnlock;
+        secretField.required = false;
         if (usingUnlock) {
           secretField.value = "";
         }