Refactor clipboard copy to use navigator.clipboard API

- Replaced deprecated `document.execCommand("copy")` with `navigator.clipboard.writeText` in `EmbedVideoModal.js`.
- Simplified fallback mechanism in `EmbedVideoModal.js` to select text and prompt user for manual copy.
- Removed `document.execCommand("copy")` fallback in `LoginModalController.js` and updated tests.
- Added unit tests for `EmbedVideoModal` copy functionality.
- Updated `LoginModalController` tests to cover NIP-46 copy button behavior.

Co-authored-by: PR0M3TH3AN <[email protected]>

js/ui/loginModalController.js

@@ -1550,11 +1550,8 @@ export default class LoginModalController {
           if (!copied && handshakeInput.select) {
             try {
               handshakeInput.select();
-              if (this.document?.execCommand) {
-                copied = this.document.execCommand("copy");
-              }
             } catch (error) {
-              copied = false;
+              // ignore
             }
           }
           if (copied) {