fix(miniapp): persist session token for WeChat login

Return sessionToken from login endpoints and store it client-side because
wx.request cannot rely on Set-Cookie. Also fix multi-cookie parsing,
start on login page, and guard chat rendering fields.

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

miniapp/app.json

@@ -1,7 +1,7 @@
 {
   "pages": [
-    "pages/chat/index",
     "pages/login/index",
+    "pages/chat/index",
     "pages/sessions/index",
     "pages/space/index",
     "pages/webview/index"