Refactor video modal sharing options into a unified Share menu

- Replaced the standalone "Copy Magnet" button and "Share" button with a new Share menu.
- Implemented `createVideoShareMenuPanel` to render the new menu with "Copy URL", "Share on Nostr", "Copy Magnet", and "Copy CDN Link" options.
- Updated `VideoModal.js` to manage the share menu state and handle actions.
- Added `video:share-nostr` and `video:copy-cdn` event handlers in `app.js`.
- Removed the `#copyMagnetBtn` element from `video-modal.html`.
- Preserved existing functionality for "Copy URL" and "Copy Magnet".
- "Share on Nostr" is currently a placeholder logging a message, disabled for logged-out users.
- "Copy CDN Link" is disabled if no URL is available.

components/video-modal.html

@@ -304,25 +304,6 @@
                     </div>
                   </div>
                   <div class="video-modal__actions-group">
-                    <button
-                      id="copyMagnetBtn"
-                      type="button"
-                      class="video-modal__action-button focus-ring accent-action-button"
-                      aria-label="Copy magnet"
-                    >
-                      <svg
-                        class="video-modal__action-icon icon-image"
-                        viewBox="0 0 24 24"
-                        aria-hidden="true"
-                        focusable="false"
-                      >
-                        <path
-                          fill="currentColor"
-                          d="M17.374 20.235c2.444-2.981 6.626-8.157 6.626-8.157l-3.846-3.092s-2.857 3.523-6.571 8.097c-4.312 5.312-11.881-2.41-6.671-6.671 4.561-3.729 8.097-6.57 8.097-6.57l-3.092-3.842s-5.173 4.181-8.157 6.621c-2.662 2.175-3.76 4.749-3.76 7.24 0 5.254 4.867 10.139 10.121 10.139 2.487 0 5.064-1.095 7.253-3.765zm4.724-7.953l-1.699 2.111-1.74-1.397 1.701-2.114 1.738 1.4zm-10.386-10.385l1.4 1.738-2.113 1.701-1.397-1.74 2.11-1.699z"
-                        />
-                      </svg>
-                      <span class="video-modal__action-label">Copy magnet</span>
-                    </button>
                     <div class="popover" data-modal-zap-wrapper>
                       <button
                         id="modalZapBtn"