feat: chat scroll/pagination, message search; fix members dialog growth

- Messages: the message list now scrolls inside a fixed-height pane instead of
  growing the page; new messages autoscroll only when already at the bottom,
  and scrolling near the top loads older history (40 at a time) with the
  position preserved.
- Add a message search box in the conversations sidebar backed by a new
  GET /api/v1/messages/search endpoint (searches the caller's conversations,
  returns snippets); clicking a result opens that conversation.
- Manage-members dialog no longer grows horizontally: results are stacked in a
  fixed-width, scrollable list.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

web/templates/messages.html

@@ -6,7 +6,9 @@
       <h2>Messages</h2>
       <button class="btn small primary" id="conv-new">＋ New</button>
     </div>
+    <input type="search" id="conv-search" class="mt" placeholder="Search messages…" aria-label="Search messages">
     <ul id="conv-list" class="conv-list"></ul>
+    <ul id="search-results" class="conv-list" hidden></ul>
   </aside>
   <section class="chat-main card">
     <div class="spread">