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/sitesmith/SitesmithButton.tsx

@@ -21,8 +21,8 @@ export const SitesmithButton: React.FC<Props> = ({ onClick }) => {
         color: '#fff', border: 'none', padding: '6px 12px', borderRadius: 6, cursor: 'pointer', fontWeight: 500,
       }}
     >
-      ✨ Sitesmith
-      {locked && <span aria-hidden style={{ marginLeft: 6, fontSize: 12 }}>🔒</span>}
+      <i className="fa fa-magic" aria-hidden style={{ marginRight: 6 }} /> Sitesmith
+      {locked && <i className="fa fa-lock" aria-hidden style={{ marginLeft: 6, fontSize: 12 }} />}
       {capped && !locked && <span aria-hidden style={{ marginLeft: 6, fontSize: 11, opacity: 0.85 }}>(cap)</span>}
     </button>
   );