fix(preview): harden sandbox and transformation

tests/test_smoke.py

@@ -592,3 +592,17 @@ def test_render_hint_is_only_added_when_the_tool_is_offered():
     from synapse import tools as t
     assert t.wants_render_preview("draw me a chart")
     assert not t.wants_render_preview("Summarize what a thread-safe LRU cache needs")
+
+
+def test_preview_iframe_cannot_navigate_to_a_network_url():
+    """The child CSP blocks resource loads; the parent CSP must separately
+    block a sandboxed frame from navigating its own browsing context."""
+    index = (REPO_ROOT / "interface" / "web" / "index.html").read_text(encoding="utf-8")
+    markdown = (REPO_ROOT / "interface" / "web" / "src" / "Markdown.jsx").read_text(
+        encoding="utf-8"
+    )
+    assert "frame-src data:" in index
+    assert 'sandbox="allow-scripts"' in markdown
+    assert "encodeURIComponent(doc)" in markdown
+    assert "src={frameUrl}" in markdown
+    assert "srcDoc={doc}" not in markdown