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/components/EmbedVideoModal.js

@@ -414,19 +414,9 @@ export class EmbedVideoModal {
     textarea.select();
     textarea.setSelectionRange(0, textarea.value.length);
 
-    let succeeded = false;
-    try {
-      succeeded = this.document.execCommand("copy");
-    } catch (error) {
-      logger.user.warn("Embed modal copy fallback failed.", error);
-      succeeded = false;
-    }
-
-    if (succeeded) {
-      this.callbacks.showSuccess("Embed code copied to clipboard!");
-    } else {
-      this.callbacks.showError("Unable to copy embed code. Please copy it manually.");
-    }
+    this.callbacks.showError(
+      "Unable to copy embed code. Please copy it manually."
+    );
   }
 
   async open({ video, triggerElement } = {}) {