feat(web): add prompt editor with split-pane and Markdown editing

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

web/client/src/hooks/usePrompts.ts

@@ -0,0 +1,25 @@
+import { useState, useCallback } from 'react';
+import { api } from '@/lib/api';
+
+export function usePrompts() {
+  const [content, setContent] = useState<string>('');
+  const [path, setPath] = useState<string>('');
+  const [loading, setLoading] = useState(false);
+
+  const load = useCallback(async (accountId: string, type: string) => {
+    setLoading(true);
+    try {
+      const result = await api.getPrompt(accountId, type);
+      setContent(result.content);
+      setPath(result.path);
+    } catch { setContent(''); setPath(''); }
+    setLoading(false);
+  }, []);
+
+  const save = useCallback(async (accountId: string, type: string, newContent: string) => {
+    await api.savePrompt(accountId, type, newContent);
+    setContent(newContent);
+  }, []);
+
+  return { content, path, loading, load, save, setContent };
+}