fix(messages): don't inherit a stale chat thread across account switch

Switching accounts does location.reload(), which preserves history.state — so the new
account re-mounted the Messages module from the previous account's _sub sub-view, whose
partnerId belongs to the OLD identity (often the new account's own id → an empty
self-thread). Symptom: after switching to the recipient you land on a blank thread and
the just-received message never shows (F5 keeps history.state); only going back to the
list and reopening the real conversation works.

Fix: (1) strip _sub/_ov from history.state before the switch reload so the new account
starts at the module root; (2) guard Messages so a thread view with partnerId === meId
falls back to the conversation list (can't message yourself).

frontend/src/components/Messages.tsx

@@ -22,7 +22,9 @@ export default function Messages({ meId }: { meId: number }) {
   // the module (open() pushes an entry; back() = history.back()).
   const { view, open, back } = useHistorySubview<View>("list");
 
-  if (typeof view === "object") {
+  // Guard against a stale sub-view whose partnerId is the current user's own id (you can't
+  // message yourself). This can be inherited across an account switch — fall back to the list.
+  if (typeof view === "object" && view.partnerId !== meId) {
     return (
       <PageThread
         meId={meId}