feat: add web-based Gateway Dashboard UI

## Summary

Adds a built-in web dashboard to the PicoClaw gateway, accessible at `http://<host>:<port>/dashboard` with no additional dependencies or configuration required.

The dashboard is served by the existing gateway HTTP server and embedded directly into the binary.

## Changes

### New: `pkg/dashboard/`
- `server.go` — REST API handlers for all dashboard endpoints
- `ui.go` — Single-file SPA frontend embedded as a Go string constant

### Modified: `pkg/channels/manager.go`
- Added `Mux() *http.ServeMux` method to expose the shared HTTP mux to external packages

### Modified: `cmd/picoclaw/internal/gateway/helpers.go`
- Initialize and register the dashboard server on gateway startup

## Features

| Section | Description |
|---------|-------------|
| Overview | Live stats: agents, skills, cron jobs, sessions |
| Config Editor | Read/write `config.json` directly from the browser |
| Agents | View configured agents, model, workspace, linked skills |
| SOUL.md Editor | Edit agent personality directly from the browser |
| Skills | Browse, create, edit, and delete skills with inline SKILL.md editor |
| Cron Jobs | Create, enable/disable, and delete scheduled jobs |
| Sessions | View active session files with size and timestamp |
| Logs | Tail last 200 lines of the gateway log |

## API Endpoints
```
GET/PUT  /api/config
GET      /api/agents
GET/PUT  /api/soul
GET/PUT/POST/DELETE /api/skill?name=X
GET      /api/skills
GET/POST/PUT/DELETE /api/cron
GET      /api/sessions
GET      /api/logs
GET      /api/status
```

## Notes

- No authentication (designed for local network / self-hosted use)
- No new dependencies — uses only Go standard library
- Frontend is a single-file SPA with no build step required
- Dashboard auto-refreshes uptime every 10 seconds
- Config and SOUL.md edits take effect on next agent interaction

cmd/picoclaw/internal/gateway/helpers.go

@@ -16,7 +16,6 @@ import (
 	_ "github.com/sipeed/picoclaw/pkg/channels/dingtalk"
 	_ "github.com/sipeed/picoclaw/pkg/channels/discord"
 	_ "github.com/sipeed/picoclaw/pkg/channels/feishu"
-	_ "github.com/sipeed/picoclaw/pkg/channels/irc"
 	_ "github.com/sipeed/picoclaw/pkg/channels/line"
 	_ "github.com/sipeed/picoclaw/pkg/channels/maixcam"
 	_ "github.com/sipeed/picoclaw/pkg/channels/onebot"
@@ -31,6 +30,7 @@ import (
 	"github.com/sipeed/picoclaw/pkg/cron"
 	"github.com/sipeed/picoclaw/pkg/devices"
 	"github.com/sipeed/picoclaw/pkg/health"
+	"github.com/sipeed/picoclaw/pkg/dashboard"
 	"github.com/sipeed/picoclaw/pkg/heartbeat"
 	"github.com/sipeed/picoclaw/pkg/logger"
 	"github.com/sipeed/picoclaw/pkg/media"
@@ -182,6 +182,10 @@ func gatewayCmd(debug bool) error {
 	addr := fmt.Sprintf("%s:%d", cfg.Gateway.Host, cfg.Gateway.Port)
 	channelManager.SetupHTTPServer(addr, healthServer)
 
+	dashServer := dashboard.NewServer(cfg, cronService, internal.GetConfigPath())
+	dashServer.RegisterOnMux(channelManager.Mux())
+	fmt.Printf("✓ Dashboard available at http://%s:%d/dashboard\n", cfg.Gateway.Host, cfg.Gateway.Port)
+
 	if err := channelManager.StartAll(ctx); err != nil {
 		fmt.Printf("Error starting channels: %v\n", err)
 		return err