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/components/Icon.tsx

@@ -125,6 +125,12 @@ const P: Record<string, ReactNode> = {
       <path d="M15 4a3 3 0 0 1 3 3 3 3 0 0 1 2 5 3 3 0 0 1-2 5 3 3 0 0 1-6 1" />
     </>
   ),
+  search: (
+    <>
+      <circle cx="11" cy="11" r="6.5" />
+      <path d="m16 16 4.5 4.5" />
+    </>
+  ),
   eye: (
     <>
       <path d="M2 12s3.5-7 10-7 10 7 10 7-3.5 7-10 7S2 12 2 12Z" />