fix(timeline): remember 'Read more' expansion per message for the session (#152)

CollapsibleBody kept its collapsed flag in component state, which the
timeline's windowed rendering throws away when a row unmounts. A module-level
Set<eventId> (never persisted, cleared on reload) now seeds the state, so
scrolling away and back, or a re-render, keeps an expanded message open.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PPmy3tPq869XDW4njjVaKA

src/app/components/message/MsgTypeRenderers.tsx

@@ -46,14 +46,31 @@ type CollapsibleBodyProps = {
   eventId?: string;
   children: ReactNode;
 };
+// [Gitea #152] Messages the user expanded this session. The timeline unmounts
+// rows as they scroll away, so component state alone forgot the choice;
+// module-scoped (never persisted) survives remounts and clears on reload.
+const expandedMessages = new Set<string>();
+
 function CollapsibleBody({ eventId, children }: CollapsibleBodyProps) {
   const bodyRef = useRef<HTMLDivElement>(null);
   const [needsCollapse, setNeedsCollapse] = useState(false);
-  const [collapsed, setCollapsed] = useState(true);
+  const [collapsed, setCollapsedState] = useState(
+    () => !(eventId && expandedMessages.has(eventId)),
+  );
+  const setCollapsed = (next: boolean | ((c: boolean) => boolean)) => {
+    setCollapsedState((c) => {
+      const value = typeof next === 'function' ? next(c) : next;
+      if (eventId) {
+        if (value) expandedMessages.delete(eventId);
+        else expandedMessages.add(eventId);
+      }
+      return value;
+    });
+  };
 
   // Reset collapsed state when the event changes (new message)
   useEffect(() => {
-    setCollapsed(true);
+    setCollapsedState(!(eventId && expandedMessages.has(eventId)));
     setNeedsCollapse(false);
   }, [eventId]);