sitesmith: null-safe esc() in Navbar/Menu/Logo + clear chat button

The prior null-safe esc patch only matched 'const esc =' declarations;
Menu/Navbar/Logo use 'function esc(str: string)' syntax and slipped
through. Patched those three to coerce non-strings the same way.

Added "Clear chat" button in the modal header that appears when there's
any message history. Confirms with the user before posting to the new
clear_history endpoint, which deletes all messages + the thread row
for the current site (usage rows are preserved for billing).

craft/src/hooks/useSitesmith.ts

@@ -49,5 +49,22 @@ export function useSitesmith(siteId: number) {
     return j;
   }, [whpConfig, siteId, fetchHistory, refreshEntitlement]);
 
-  return { summary, messages, loading, error, send, refreshEntitlement };
+  const clearHistory = useCallback(async (): Promise<{ ok: boolean; cleared?: number; error?: string }> => {
+    if (!whpConfig) return { ok: false, error: 'No WHP config' };
+    try {
+      const r = await fetch(`${apiBase(whpConfig.apiUrl)}?action=clear_history`, {
+        method: 'POST',
+        credentials: 'include',
+        headers: { 'Content-Type': 'application/json', 'X-CSRF-Token': whpConfig.csrfToken },
+        body: JSON.stringify({ site_id: siteId }),
+      });
+      const j = await r.json();
+      if (j.ok) setMessages([]);
+      return j;
+    } catch (e: any) {
+      return { ok: false, error: String(e?.message ?? e) };
+    }
+  }, [whpConfig, siteId]);
+
+  return { summary, messages, loading, error, send, refreshEntitlement, clearHistory };
 }