Fix feed error response rendering with to_xml

The 422 error handlers were using str() to convert FT objects to HTML,
which produces Python repr output instead of HTML. Changed to use
to_xml() like other routes (eggs.py, products.py) do.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/animaltrack/web/routes/feed.py

@@ -6,7 +6,7 @@ from __future__ import annotations
 import time
 from typing import Any
 
-from fasthtml.common import APIRouter, add_toast
+from fasthtml.common import APIRouter, add_toast, to_xml
 from starlette.requests import Request
 from starlette.responses import HTMLResponse
 
@@ -437,7 +437,7 @@ def _render_give_error(
         HTMLResponse with 422 status.
     """
     return HTMLResponse(
-        content=str(
+        content=to_xml(
             render_page(
                 request,
                 feed_page(
@@ -471,7 +471,7 @@ def _render_purchase_error(request, locations, feed_types, error_message):
         HTMLResponse with 422 status.
     """
     return HTMLResponse(
-        content=str(
+        content=to_xml(
             render_page(
                 request,
                 feed_page(