Replace emoji-as-icons in editor chrome with Font Awesome

Unicode emoji/glyphs (Sitesmith's sparkle, lock, close X) render as
tofu on systems without an emoji font. Swap for the FA4 glyphs the
rest of the chrome already uses:
- SitesmithButton/SitesmithModal: sparkle -> fa-magic, lock -> fa-lock
- ContextMenu "Ask Sitesmith" entry: sparkle -> fa-magic (via new
  optional MenuItem.icon field)
- TemplateModal/SitesmithModal close buttons, PagesPanel delete,
  AssetsPanel delete/cancel: ✕ -> fa-times / fa-trash

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/panels/topbar/TemplateModal.tsx

@@ -248,7 +248,7 @@ export const TemplateModal: React.FC<TemplateModalProps> = ({ open, onClose }) =
             </p>
           </div>
           <button onClick={onClose} style={closeButtonStyle} title="Close">
-            &#10005;
+            <i className="fa fa-times" aria-hidden />
           </button>
         </div>