feat(tailscale): add connect/disconnect, exit-node and LAN-access controls (#2644)

* feat(tailscale): add connect/disconnect/exit-node/LAN-access backend

The Tailscale backend previously exposed only read-only status
(tailscale.getStatus, tailscale.refresh). This adds write actions through the
existing tailscale.com/client/local integration:

- tailscale.connect / tailscale.disconnect (EditPrefs WantRunning)
- tailscale.setExitNode (EditPrefs ExitNodeID; empty id clears it and any
  legacy ExitNodeIP, mirroring `tailscale set --exit-node`)
- tailscale.setAllowLanAccess (EditPrefs ExitNodeAllowLANAccess)

The manager's client interface gains GetPrefs/EditPrefs; fetchState merges
ExitNodeAllowLANAccess from prefs, and Peer exposes ExitNodeOption so the UI
can list exit-node-capable peers.

* feat(tailscale): expose the new actions in TailscaleService

Adds connectTailscale/disconnectTailscale, setExitNode/clearExitNode and
setAllowLanAccess wrappers, plus derived exitNodeOptions/currentExitNode and the
exitNodeAllowLanAccess state. Write-action errors surface via ToastService.

* feat(tailscale): add connection, exit-node and LAN-access controls to the widget

The control-center widget toggle was a no-op. It now connects/disconnects, and
the detail panel gains a connection status row with a connect/disconnect button,
an exit-node picker and a LAN-access toggle.

core/internal/server/tailscale/client.go

@@ -66,16 +66,17 @@ func convertPeerStatus(ps *ipnstate.PeerStatus, users map[tailcfg.UserID]tailcfg
 	}
 
 	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,
+		ID:             string(ps.ID),
+		Hostname:       hostname,
+		DNSName:        dnsName,
+		OS:             ps.OS,
+		Online:         ps.Online,
+		Active:         ps.Active,
+		ExitNode:       ps.ExitNode,
+		ExitNodeOption: ps.ExitNodeOption,
+		Relay:          ps.Relay,
+		RxBytes:        ps.RxBytes,
+		TxBytes:        ps.TxBytes,
 	}
 
 	for _, ip := range ps.TailscaleIPs {