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).

components/login-modal.html

@@ -204,7 +204,6 @@
                 spellcheck="false"
                 autocomplete="off"
                 data-nsec-secret
-                required
               ></textarea>
               <label class="inline-flex items-center gap-2 text-sm text-text">
                 <input type="checkbox" class="checkbox" data-nsec-remember />