Add browser_fetch helpers and async HTTP demos for Pyodide

Ship pyfetch-based fetch utilities in lib/, run asyncio scripts via
nest-asyncio in the worker, and add sample demos for HTTPS in the browser.

Co-authored-by: Cursor <cursoragent@cursor.com>

lib/browser_fetch.py

@@ -0,0 +1,38 @@
+"""Browser-friendly async HTTP for the Pyodide worker.
+
+These helpers use ``pyodide.http.pyfetch``, which maps to the browser
+``fetch`` API (no Python TLS stack). Prefer them for ``https://`` in
+Pyodide: ``aiohttp`` in the worker often raises
+``RuntimeError('SSL is not supported.')`` for HTTPS even though the wheel
+exists, because user-level SSL is not wired the same as on CPython.
+
+The browser's normal rules apply: the page is served over HTTPS so
+``http://`` URLs are blocked as mixed content, and the response host must
+send permissive CORS headers (e.g. ``Access-Control-Allow-Origin``) or the
+browser hides the body even if the request succeeded.
+"""
+
+from __future__ import annotations
+
+from typing import Any
+
+
+async def fetch_text(url: str) -> str:
+    from pyodide.http import pyfetch
+
+    r = await pyfetch(url)
+    return await r.text()
+
+
+async def fetch_bytes(url: str) -> bytes:
+    from pyodide.http import pyfetch
+
+    r = await pyfetch(url)
+    return await r.bytes()
+
+
+async def fetch_json(url: str, **kwargs: Any) -> Any:
+    from pyodide.http import pyfetch
+
+    r = await pyfetch(url)
+    return await r.json(**kwargs)