Say a missing folder is missing, not empty

A folder id the account does not have rendered the ordinary empty state --
"Nothing here. This folder is empty." That is a claim about a folder that
is not there, so a stale link read as a folder that had emptied itself
rather than one that was gone (#111).

It now goes to the inbox and says why. Inbox is the kinder landing than a
dead end for a bookmark that has outlived its folder, but swapping one
folder for another without a word would be its own small lie, so it does
not do that either.

The condition worth writing a test around is not the unknown id, it is
the one guarding it. The folder list arrives after the first paint, so for
a moment *every* id is unknown, the right one included. Without that gate
this redirects on every cold load, from the folder the reader actually
asked for, and looks exactly like a flaky link -- a worse bug than the one
being fixed and a harder one to see. `isUnknownMailbox` is a small pure
function so that case can be pinned down rather than reasoned about.

Only ever reachable from outside the app, which is why it went unnoticed:
the sidebar links to ids that exist. A bookmark to a deleted folder, or a
folder link passed between accounts, is where it bites.

Verified against the mock: an unknown id lands on the inbox with the
message and a full list rather than an empty one, and a cold load straight
into a real folder stays in that folder with nothing said.

Closes #111.

web/src/lib/mailboxRoute.ts

@@ -0,0 +1,27 @@
+import type { Id, Mailbox } from "@/jmap/types";
+
+/**
+ * Whether the folder in the address is one this account does not have.
+ *
+ * Rendering it as an empty folder was the bug (#111): "Nothing here. This
+ * folder is empty" is a claim about a folder that is not there, so a stale link
+ * read as a folder that had emptied itself rather than one that was gone.
+ *
+ * The condition that matters is `loaded`. The folder list arrives after the
+ * first paint, so for a moment every id is unknown -- including the right one.
+ * Without that gate this answers true on every cold load and sends the reader
+ * to their inbox from the folder they asked for, which is a worse bug than the
+ * one it fixes and would look exactly like a flaky link.
+ */
+export function isUnknownMailbox(args: {
+  mailboxId: Id | undefined;
+  mailboxes: Record<Id, Mailbox>;
+  loaded: boolean;
+  search?: boolean;
+}): boolean {
+  const { mailboxId, mailboxes, loaded, search } = args;
+  if (search) return false;
+  if (!mailboxId) return false;
+  if (!loaded) return false;
+  return !mailboxes[mailboxId];
+}