Open the bot last visited, else the coordinator

`/admin/bots` sent the super admin to the coordinator and everyone else to
their latest bot, which lost the thread each time the tab was reopened. The
conversation pages now record where the browser is in two HttpOnly cookies
(`cobble_last_visited_bot_id`, `..._workspace_id`, a year, `Secure` only over
HTTPS), and the landing page opens that conversation again, in its workspace
when it was one.

A stale cookie can't loop: `grabReachableConversation` re-checks it with
`grabUserConversation`, the gate the conversation pages use, so a revoked
grant, a deleted bot or an archived workspace falls back to the coordinator
(super admin) or the latest bot, first workspace, empty page. Signing out
clears both cookies so the next user on that browser doesn't land in the
previous one's conversation.

src/utils/read-last-visited-bot.ts

@@ -0,0 +1,20 @@
+import { AppData } from "@/src/data/app-data";
+import { getCookie } from "./cookies";
+import parseLastVisitedBot, { type LastVisitedBot } from "./parse-last-visited-bot";
+
+type Params = {
+    req: Request;
+};
+
+/**
+ * The conversation this browser was last in, from the last-visited cookies
+ * (see `lastVisitedBotCookies`). Null when it has none, or when they don't
+ * name a conversation. Whether the user still reaches it is the caller's
+ * check: a stale grant must fall back, never redirect.
+ */
+export default function readLastVisitedBot({ req }: Params): LastVisitedBot | null {
+    return parseLastVisitedBot({
+        bot_id: getCookie(req, AppData["LastVisitedBotCookieName"]),
+        workspace_id: getCookie(req, AppData["LastVisitedWorkspaceCookieName"]),
+    });
+}