feat: add Slack-like channel and DM messaging UX

- Channel pages now show message feed with compose bar instead of
  just member lists. Messages display with agent avatars, names,
  and timestamps. Collapsible info panel shows channel details.
- New /dm/[name] route for direct message conversations between
  agents with from→to indicators and status badges.
- Sidebar DM links now navigate to /dm/{name} instead of agent
  edit forms.
- Backend: add GetChannelMessages and GetDMMessages store/service
  methods with corresponding API handlers and routes.
- Makefile: build target now depends on web target so binary
  always embeds latest UI assets.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/api/channels_handler.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"log/slog"
 	"net/http"
+	"strconv"
 
 	"github.com/go-chi/chi/v5"
 
@@ -198,6 +199,41 @@ func (h *ChannelsHandler) JoinChannel(w http.ResponseWriter, r *http.Request) {
 	writeJSON(w, http.StatusOK, map[string]string{"status": "joined"})
 }
 
+// ChannelMessages handles GET /api/channels/{name}/messages.
+func (h *ChannelsHandler) ChannelMessages(w http.ResponseWriter, r *http.Request) {
+	_, ok := OwnerIDFromContext(r.Context())
+	if !ok {
+		writeJSON(w, http.StatusUnauthorized, errorBody("unauthorized", "Authentication required"))
+		return
+	}
+
+	name := chi.URLParam(r, "name")
+	ch, err := h.channelService.GetChannelByName(r.Context(), name)
+	if err != nil {
+		writeJSON(w, http.StatusNotFound, errorBody("not_found", "Channel not found"))
+		return
+	}
+
+	limit := 100
+	if l := r.URL.Query().Get("limit"); l != "" {
+		if parsed, err := strconv.Atoi(l); err == nil && parsed > 0 {
+			limit = parsed
+		}
+	}
+
+	msgs, err := h.msgService.GetChannelMessages(r.Context(), ch.ID, limit)
+	if err != nil {
+		h.logger.Error("get channel messages failed", "error", err)
+		writeJSON(w, http.StatusInternalServerError, errorBody("server_error", "Failed to get messages"))
+		return
+	}
+
+	writeJSON(w, http.StatusOK, map[string]any{
+		"messages": msgs,
+		"total":    len(msgs),
+	})
+}
+
 // LeaveChannel handles POST /api/channels/{name}/leave.
 func (h *ChannelsHandler) LeaveChannel(w http.ResponseWriter, r *http.Request) {
 	ownerID, ok := OwnerIDFromContext(r.Context())

internal/api/messages_handler.go

@@ -425,6 +425,51 @@ func (h *MessagesHandler) GetReplies(w http.ResponseWriter, r *http.Request) {
 	})
 }
 
+// DMMessages handles GET /api/agents/{name}/messages — returns DM messages with a specific agent.
+func (h *MessagesHandler) DMMessages(w http.ResponseWriter, r *http.Request) {
+	ownerID, ok := OwnerIDFromContext(r.Context())
+	if !ok {
+		writeJSON(w, http.StatusUnauthorized, errorBody("unauthorized", "Authentication required"))
+		return
+	}
+
+	peerAgent := chi.URLParam(r, "name")
+
+	ownedAgents, err := h.agentService.ListAgents(r.Context(), ownerID)
+	if err != nil {
+		h.logger.Error("list agents failed", "error", err)
+		writeJSON(w, http.StatusInternalServerError, errorBody("server_error", "Failed to list agents"))
+		return
+	}
+
+	if len(ownedAgents) == 0 {
+		writeJSON(w, http.StatusOK, map[string]any{"messages": []*messaging.Message{}, "total": 0})
+		return
+	}
+
+	agentNames := make([]string, len(ownedAgents))
+	for i, a := range ownedAgents {
+		agentNames[i] = a.Name
+	}
+
+	limit, _ := strconv.Atoi(r.URL.Query().Get("limit"))
+	if limit <= 0 {
+		limit = 100
+	}
+
+	msgs, err := h.msgService.GetDMMessages(r.Context(), agentNames, peerAgent, limit)
+	if err != nil {
+		h.logger.Error("get dm messages failed", "error", err)
+		writeJSON(w, http.StatusInternalServerError, errorBody("server_error", "Failed to get messages"))
+		return
+	}
+
+	writeJSON(w, http.StatusOK, map[string]any{
+		"messages": msgs,
+		"total":    len(msgs),
+	})
+}
+
 func (h *MessagesHandler) isAgentOwnedBy(r *http.Request, agentName string, ownerID int64) bool {
 	if agentName == "" {
 		return false

internal/api/router.go

@@ -100,6 +100,7 @@ func NewRouterWithConfig(cfg RouterConfig) chi.Router {
 			r.Post("/api/agents", agentsHandler.RegisterAgent)
 			r.Delete("/api/agents/{name}", agentsHandler.DeleteAgent)
 			r.Post("/api/agents/{name}/revoke-key", agentsHandler.RevokeKey)
+			r.Get("/api/agents/{name}/messages", messagesHandler.DMMessages)
 		})
 
 		// API Keys
@@ -124,6 +125,7 @@ func NewRouterWithConfig(cfg RouterConfig) chi.Router {
 				r.Get("/api/channels", channelsHandler.ListChannels)
 				r.Get("/api/channels/{name}", channelsHandler.GetChannel)
 				r.Post("/api/channels", channelsHandler.CreateChannel)
+				r.Get("/api/channels/{name}/messages", channelsHandler.ChannelMessages)
 				r.Post("/api/channels/{name}/join", channelsHandler.JoinChannel)
 				r.Post("/api/channels/{name}/leave", channelsHandler.LeaveChannel)
 			})