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/SitesmithModal.tsx

@@ -103,7 +103,10 @@ export const SitesmithModal: React.FC<Props> = ({ onClose, target }) => {
     >
       <div style={panel}>
         <div style={header}>
-          <div style={{ fontWeight: 600, color: '#fff' }}>✨ Sitesmith</div>
+          <div style={{ fontWeight: 600, color: '#fff' }}>
+            <i className="fa fa-magic" aria-hidden style={{ marginRight: 6 }} />
+            Sitesmith
+          </div>
           {summary && summary.enabled && (
             <div style={{ fontSize: 12, color: '#a1a1aa', marginLeft: 16 }}>
               {summary.monthly_used} / {summary.monthly_cap} this month
@@ -136,7 +139,7 @@ export const SitesmithModal: React.FC<Props> = ({ onClose, target }) => {
               </button>
             )
           )}
-          <button onClick={onClose} aria-label="Close" style={closeBtn}>✕</button>
+          <button onClick={onClose} aria-label="Close" style={closeBtn}><i className="fa fa-times" aria-hidden /></button>
         </div>
         <div style={body}>
           <UpgradeBanner summary={summary} />