Long chats stay fast; search in a chat; media overview

Performance (measured with 5,900 messages: open 9 s -> 0.7 s, typing 440 ms -> 25 ms)
- GET /sessions/{id}?limit=N loads the end of a chat (from the last summary
  on, at most N); /sessions/{id}/messages?before|after|around loads more
  (scrolling up stops at a summary unless asked). Without parameters the
  full history as before (terminal client).
- Chat view: memoised message list (typing, streaming and status changes no
  longer re-render the history), finished turns fold into "N steps · M files"
  with the answer visible, tool card bodies and summaries are built only when
  opened, scroll-to-bottom only when content changes.

Search: /sessions/{id}/search over the whole chat; a hit loads its part
(around) and marks the message, "back to the latest messages" returns.

Media: /sessions/{id}/media lists images, videos and audio of a chat and its
subagents (generated images, tool output, files written, media in answers,
attachments); ⋯ -> Media shows them with filters, a large view and a jump to
the message.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/src/whatsnew.ts

@@ -3,6 +3,10 @@
  * under "unreleased"; scripts/release.sh renames the key to the release version.
  */
 export const WHATS_NEW: Record<string, { en: string[]; de: string[] }> = {
+  unreleased: {
+    en: ["Long chats open and react much faster: older messages load when you scroll up, finished turns fold into one line", "Search in a chat (magnifier in the top bar) and ⋯ → Media: all images, videos and audio of a chat"],
+    de: ["Lange Chats öffnen und reagieren viel schneller: ältere Nachrichten laden beim Hochscrollen, abgeschlossene Turns sind eingeklappt", "Suche im Chat (Lupe oben) und ⋯ → Medien: alle Bilder, Videos und Audiodateien eines Chats"],
+  },
   "202611-00018": {
     en: [
       "Modes with a review: every completed todo is reviewed, and the agent keeps working until the todo list is done – for overnight runs",