feat(tailscale): add Tailscale control center widget (#1875)

* feat(tailscale): add Tailscale control center widget

Full-stack Tailscale integration for DMS control center:

Backend (Go):
- Event-driven manager via WatchIPNBus (no polling)
- Reconnects with exponential backoff when tailscaled unavailable
- Typed conversion from ipnstate.Status to QML-friendly IPC types
- Testable via tailscaleClient interface with mock watcher
- Manager cleanup in cleanupManagers()
- 19 unit tests

Frontend (QML):
- TailscaleService with WebSocket subscription
- TailscaleWidget with peer list, filter chips, search
- Copy-to-clipboard for IPs and DNS names
- Daemon lifecycle handling (offline/stopped states)

Dependencies:
- Add tailscale.com v1.96.1 (official local API client)
- Bump Go to 1.26.1 (required by tailscale.com)

* cleanups

---------

Co-authored-by: bbedward <bbedward@gmail.com>

core/internal/server/tailscale/handlers_test.go

@@ -0,0 +1,97 @@
+package tailscale
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"net"
+	"testing"
+	"time"
+
+	"github.com/AvengeMedia/DankMaterialShell/core/internal/server/models"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"tailscale.com/ipn"
+	"tailscale.com/ipn/ipnstate"
+)
+
+type mockConn struct {
+	*bytes.Buffer
+}
+
+func (m *mockConn) Close() error                       { return nil }
+func (m *mockConn) LocalAddr() net.Addr                { return nil }
+func (m *mockConn) RemoteAddr() net.Addr               { return nil }
+func (m *mockConn) SetDeadline(t time.Time) error      { return nil }
+func (m *mockConn) SetReadDeadline(t time.Time) error  { return nil }
+func (m *mockConn) SetWriteDeadline(t time.Time) error { return nil }
+
+func handlerTestManager() *Manager {
+	client := &mockClient{
+		watchFn: func(ctx context.Context, mask ipn.NotifyWatchOpt) (ipnBusWatcher, error) {
+			<-ctx.Done()
+			return nil, ctx.Err()
+		},
+		statusFn: func(ctx context.Context) (*ipnstate.Status, error) {
+			return runningStatus(), nil
+		},
+	}
+	m := newManager(client)
+	m.RefreshState()
+	return m
+}
+
+func TestHandleGetStatus(t *testing.T) {
+	m := handlerTestManager()
+	defer m.Close()
+
+	buf := &bytes.Buffer{}
+	conn := &mockConn{Buffer: buf}
+
+	req := models.Request{ID: 1, Method: "tailscale.getStatus"}
+	handleGetStatus(conn, req, m)
+
+	var resp models.Response[TailscaleState]
+	err := json.NewDecoder(buf).Decode(&resp)
+	require.NoError(t, err)
+	assert.Equal(t, 1, resp.ID)
+	assert.NotNil(t, resp.Result)
+	assert.True(t, resp.Result.Connected)
+	assert.Equal(t, "cachyos", resp.Result.Self.Hostname)
+}
+
+func TestHandleRefresh(t *testing.T) {
+	m := handlerTestManager()
+	defer m.Close()
+
+	buf := &bytes.Buffer{}
+	conn := &mockConn{Buffer: buf}
+
+	req := models.Request{ID: 1, Method: "tailscale.refresh"}
+	handleRefresh(conn, req, m)
+
+	var resp models.Response[models.SuccessResult]
+	err := json.NewDecoder(buf).Decode(&resp)
+	require.NoError(t, err)
+	assert.Equal(t, 1, resp.ID)
+	assert.NotNil(t, resp.Result)
+	assert.True(t, resp.Result.Success)
+}
+
+func TestHandleRequest_UnknownMethod(t *testing.T) {
+	m := handlerTestManager()
+	defer m.Close()
+
+	buf := &bytes.Buffer{}
+	conn := &mockConn{Buffer: buf}
+
+	req := models.Request{ID: 1, Method: "tailscale.unknownMethod"}
+	HandleRequest(conn, req, m)
+
+	var resp models.Response[any]
+	err := json.NewDecoder(buf).Decode(&resp)
+	require.NoError(t, err)
+	assert.Nil(t, resp.Result)
+	assert.NotEmpty(t, resp.Error)
+	assert.Contains(t, resp.Error, "unknown method")
+}