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>

internal/http/messages.go

@@ -29,6 +29,7 @@ func (s *Server) routesMessages(mux *http.ServeMux) {
 	mux.Handle("DELETE /api/v1/conversations/{id}/members/{userId}", s.authed(s.handleRemoveMember))
 	mux.Handle("POST /api/v1/files", s.authed(s.handleUploadFile))
 	mux.Handle("GET /api/v1/users", s.requireAuth(http.HandlerFunc(s.handleSearchUsers)))
+	mux.Handle("GET /api/v1/messages/search", s.requireAuth(http.HandlerFunc(s.handleSearchMessages)))
 }
 
 // conversation loads + authorizes participant access.
@@ -414,6 +415,117 @@ func (s *Server) handleSearchUsers(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, map[string]any{"users": out})
 }
 
+// handleSearchMessages finds messages containing the query within the caller's
+// own conversations, newest first.
+func (s *Server) handleSearchMessages(w http.ResponseWriter, r *http.Request) {
+	me := CurrentUser(r.Context())
+	q := strings.TrimSpace(r.URL.Query().Get("q"))
+	if len([]rune(q)) < 2 {
+		writeJSON(w, http.StatusOK, map[string]any{"results": []any{}})
+		return
+	}
+	convs, err := s.store.ListConversations(r.Context(), me.ID)
+	if err != nil {
+		s.internalError(w, r, "search: conversations", err)
+		return
+	}
+	if len(convs) == 0 {
+		writeJSON(w, http.StatusOK, map[string]any{"results": []any{}})
+		return
+	}
+	ids := make([]string, 0, len(convs))
+	titles := make(map[string]string, len(convs))
+	for _, c := range convs {
+		ids = append(ids, c.ID)
+		title := c.Title
+		if c.Kind == "dm" {
+			for _, pid := range c.ParticipantIDs {
+				if pid != me.ID {
+					if u, err := s.store.UserByID(r.Context(), pid); err == nil {
+						title = u.Name
+					}
+				}
+			}
+		}
+		if title == "" {
+			title = "Conversation"
+		}
+		titles[c.ID] = title
+	}
+	cur, err := s.store.DB.Collection("messages").Find(r.Context(), bson.M{
+		"conversationId": bson.M{"$in": ids},
+		"deletedAt":      nil,
+		"body":           bson.M{"$regex": regexEscape(q), "$options": "i"},
+	}, options.Find().SetSort(bson.D{{Key: "_id", Value: -1}}).SetLimit(30))
+	if err != nil {
+		s.internalError(w, r, "search messages", err)
+		return
+	}
+	var msgs []store.Message
+	if err := cur.All(r.Context(), &msgs); err != nil {
+		s.internalError(w, r, "decode search", err)
+		return
+	}
+	results := make([]map[string]any, 0, len(msgs))
+	for _, m := range msgs {
+		results = append(results, map[string]any{
+			"conversationId":    m.ConversationID,
+			"conversationTitle": titles[m.ConversationID],
+			"messageId":         m.ID,
+			"senderId":          m.SenderID,
+			"snippet":           snippetAround(stripTags(m.Body), q),
+		})
+	}
+	writeJSON(w, http.StatusOK, map[string]any{"results": results})
+}
+
+// stripTags removes HTML tags and collapses whitespace for plain-text snippets.
+func stripTags(html string) string {
+	var b strings.Builder
+	inTag := false
+	for _, r := range html {
+		switch {
+		case r == '<':
+			inTag = true
+		case r == '>':
+			inTag = false
+		case !inTag:
+			b.WriteRune(r)
+		}
+	}
+	return strings.Join(strings.Fields(b.String()), " ")
+}
+
+// snippetAround returns a short text window centered on the first
+// case-insensitive match of q.
+func snippetAround(text, q string) string {
+	runes := []rune(text)
+	idx := strings.Index(strings.ToLower(text), strings.ToLower(q))
+	if idx < 0 {
+		if len(runes) > 100 {
+			return string(runes[:100]) + "…"
+		}
+		return text
+	}
+	start := len([]rune(text[:idx]))
+	from := start - 30
+	if from < 0 {
+		from = 0
+	}
+	to := start + len([]rune(q)) + 50
+	if to > len(runes) {
+		to = len(runes)
+	}
+	out := string(runes[from:to])
+	if from > 0 {
+		out = "…" + out
+	}
+	if to < len(runes) {
+		out += "…"
+	}
+	return out
+}
+
 // HandleInboundWS relays ephemeral client events (typing indicator).
 func (s *Server) HandleInboundWS(userID string, msg ws.Message) {
 	if msg.Channel != "chat" || msg.Event != "typing" {