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/client.go

@@ -0,0 +1,135 @@
+package tailscale
+
+import (
+	"fmt"
+	"sort"
+	"strings"
+	"time"
+
+	"tailscale.com/ipn/ipnstate"
+	"tailscale.com/tailcfg"
+)
+
+// convertStatus converts an ipnstate.Status into our TailscaleState IPC type.
+func convertStatus(status *ipnstate.Status) *TailscaleState {
+	connected := status.BackendState == "Running"
+
+	state := &TailscaleState{
+		Connected:    connected,
+		BackendState: status.BackendState,
+		Version:      status.Version,
+	}
+
+	if status.CurrentTailnet != nil {
+		state.TailnetName = status.CurrentTailnet.Name
+		state.MagicDNSSuffix = status.CurrentTailnet.MagicDNSSuffix
+	}
+
+	if !connected {
+		return state
+	}
+
+	users := status.User
+
+	if status.Self != nil {
+		state.Self = convertPeerStatus(status.Self, users)
+	}
+
+	if len(status.Peer) > 0 {
+		peers := make([]Peer, 0, len(status.Peer))
+		for _, ps := range status.Peer {
+			peers = append(peers, convertPeerStatus(ps, users))
+		}
+		sort.Slice(peers, func(i, j int) bool {
+			if peers[i].Online != peers[j].Online {
+				return peers[i].Online
+			}
+			return strings.ToLower(peers[i].Hostname) < strings.ToLower(peers[j].Hostname)
+		})
+		state.Peers = peers
+	}
+
+	return state
+}
+
+// convertPeerStatus converts an ipnstate.PeerStatus into our Peer IPC type.
+func convertPeerStatus(ps *ipnstate.PeerStatus, users map[tailcfg.UserID]tailcfg.UserProfile) Peer {
+	dnsName := strings.TrimSuffix(ps.DNSName, ".")
+
+	// DNSName first label is unique per node; OS HostName is not.
+	hostname := ps.HostName
+	if dnsName != "" {
+		parts := strings.SplitN(dnsName, ".", 2)
+		if len(parts) > 0 && parts[0] != "" {
+			hostname = parts[0]
+		}
+	}
+
+	peer := Peer{
+		ID:       string(ps.ID),
+		Hostname: hostname,
+		DNSName:  dnsName,
+		OS:       ps.OS,
+		Online:   ps.Online,
+		Active:   ps.Active,
+		ExitNode: ps.ExitNode,
+		Relay:    ps.Relay,
+		RxBytes:  ps.RxBytes,
+		TxBytes:  ps.TxBytes,
+	}
+
+	for _, ip := range ps.TailscaleIPs {
+		if ip.Is4() {
+			if peer.TailscaleIP == "" {
+				peer.TailscaleIP = ip.String()
+			}
+		} else {
+			if peer.TailscaleIPv6 == "" {
+				peer.TailscaleIPv6 = ip.String()
+			}
+		}
+	}
+
+	if ps.Tags != nil {
+		peer.Tags = ps.Tags.AsSlice()
+	}
+
+	if ps.UserID > 0 {
+		if user, ok := users[ps.UserID]; ok {
+			peer.Owner = user.LoginName
+		}
+	}
+
+	if !ps.LastSeen.IsZero() {
+		peer.LastSeen = formatRelativeTime(ps.LastSeen)
+	}
+
+	return peer
+}
+
+// formatRelativeTime formats a time as a human-readable relative duration (e.g., "5 minutes ago").
+func formatRelativeTime(t time.Time) string {
+	d := time.Since(t)
+	switch {
+	case d < time.Minute:
+		return "just now"
+	case d < time.Hour:
+		m := int(d.Minutes())
+		if m == 1 {
+			return "1 minute ago"
+		}
+		return fmt.Sprintf("%d minutes ago", m)
+	case d < 24*time.Hour:
+		h := int(d.Hours())
+		if h == 1 {
+			return "1 hour ago"
+		}
+		return fmt.Sprintf("%d hours ago", h)
+	default:
+		days := int(d.Hours() / 24)
+		if days == 1 {
+			return "1 day ago"
+		}
+		return fmt.Sprintf("%d days ago", days)
+	}
+}