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/static/js/messages.js

@@ -61,6 +61,7 @@ function renderConvList() {
 async function openConversation(c) {
   current = c;
   oldestCursor = '';
+  noMoreOlder = false;
   document.getElementById('chat-title').textContent = c.title;
   const mbtn = document.getElementById('chat-members');
   mbtn.hidden = !(c.kind !== 'dm' && c.creatorId && c.creatorId === meId);
@@ -74,19 +75,36 @@ async function openConversation(c) {
   history.replaceState(null, '', '/messages?c=' + c.id);
 }
 
+const MSG_PAGE = 40;
 async function loadMessages(prepend) {
   const res = await api('GET',
-    `/api/v1/conversations/${current.id}/messages?cursor=${prepend ? oldestCursor : ''}`);
+    `/api/v1/conversations/${current.id}/messages?limit=${MSG_PAGE}&cursor=${prepend ? oldestCursor : ''}`);
   const msgs = res.messages;
+  if (prepend && msgs.length < MSG_PAGE) noMoreOlder = true;
   if (msgs.length) oldestCursor = msgs[0].id;
   const nodes = await Promise.all(msgs.map(renderMessage));
-  if (prepend) msgHost.prepend(...nodes);
-  else {
+  if (prepend) {
+    // preserve the scroll position so the view doesn't jump when older
+    // messages are inserted above the current viewport
+    const before = msgHost.scrollHeight;
+    const top = msgHost.scrollTop;
+    msgHost.prepend(...nodes);
+    msgHost.scrollTop = top + (msgHost.scrollHeight - before);
+  } else {
     msgHost.append(...nodes);
     msgHost.scrollTop = msgHost.scrollHeight;
   }
 }
 
+// load older history when the user scrolls near the top
+let loadingOlder = false;
+let noMoreOlder = false;
+msgHost.addEventListener('scroll', async () => {
+  if (msgHost.scrollTop > 60 || loadingOlder || noMoreOlder || !current) return;
+  loadingOlder = true;
+  try { await loadMessages(true); } finally { loadingOlder = false; }
+});
+
 // ULID ids embed a 48-bit ms timestamp in the first 10 chars.
 function ulidTime(id) {
   const A = '0123456789ABCDEFGHJKMNPQRSTVWXYZ';
@@ -199,8 +217,9 @@ const typingNames = new Map();
 subscribe('chat', async (event, data) => {
   if (event === 'message' && data) {
     if (current && data.conversationId === current.id) {
+      const nearBottom = msgHost.scrollHeight - msgHost.scrollTop - msgHost.clientHeight < 120;
       msgHost.appendChild(await renderMessage(data));
-      msgHost.scrollTop = msgHost.scrollHeight;
+      if (nearBottom) msgHost.scrollTop = msgHost.scrollHeight;
       api('POST', `/api/v1/conversations/${current.id}/read`, {}).catch(() => {});
     } else {
       loadConversations();
@@ -299,6 +318,46 @@ document.getElementById('newconv-form').addEventListener('submit', async (e) =>
   } catch (err) { toast(err.message, 'err'); }
 });
 
+// ---- message search ----
+const convSearch = document.getElementById('conv-search');
+let msgSearchTimer = null;
+async function runMessageSearch(q) {
+  const list = document.getElementById('conv-list');
+  const results = document.getElementById('search-results');
+  if (q.length < 2) { results.hidden = true; results.replaceChildren(); list.hidden = false; return; }
+  try {
+    const res = await api('GET', `/api/v1/messages/search?q=${encodeURIComponent(q)}`);
+    list.hidden = true; results.hidden = false;
+    results.replaceChildren(...res.results.map((r) => {
+      const li = document.createElement('li');
+      const b = document.createElement('button');
+      b.type = 'button';
+      b.className = 'conv-item';
+      b.style.cssText = 'flex-direction:column;align-items:flex-start;gap:2px';
+      b.innerHTML = `<span>${esc(r.conversationTitle)}</span>
+        <span class="muted" style="font-size:0.82rem">${esc(r.snippet)}</span>`;
+      b.addEventListener('click', () => {
+        const c = conversations.find((x) => x.id === r.conversationId);
+        if (c) {
+          convSearch.value = '';
+          results.hidden = true; list.hidden = false;
+          openConversation(c);
+        }
+      });
+      li.appendChild(b);
+      return li;
+    }));
+    if (!res.results.length) {
+      results.innerHTML = '<li class="muted" style="padding:10px">No matching messages.</li>';
+    }
+  } catch (e) { /* ignore transient search errors */ }
+}
+convSearch.addEventListener('input', () => {
+  clearTimeout(msgSearchTimer);
+  const q = convSearch.value.trim();
+  msgSearchTimer = setTimeout(() => runMessageSearch(q), 250);
+});
+
 // ---- manage members (group creator only) ----
 const membersDlg = document.getElementById('members-dialog');
 let mmMemberIds = new Set();