feat(vpn): unarr vpn command + report/arbitrate the WireGuard slot

Add `unarr vpn` (status/enable/disable, with `status --check`) to manage the
managed WireGuard split-tunnel from the CLI. The daemon now reports its
split-tunnel state (active, mode, exit server) to the web on register and on
every sync, and sends its agent id when fetching the VPN config so the web can
arbitrate the single WireGuard slot (1 VPNResellers account = 1 WG keypair = 1
concurrent connection): the first agent claims it; the rest are told to run
OpenVPN on their own host (1 WireGuard + up to 9 OpenVPN = 10).

`status --check` passes probe=1 so it validates provisioning without claiming
the slot. VPNActive drops omitempty so a downed tunnel reaches the server and
frees the slot. Bumps to 0.9.2 with CHANGELOG + README VPN section.

internal/agent/sync.go

@@ -36,6 +36,10 @@ type SyncClient struct {
 	OnSyncSuccess    func() // called after each successful sync (e.g. to update state file)
 	GetFreeSlots     func() int
 	GetTaskStates    func() []TaskState // returns current state of all active + recently finished tasks
+	// GetVPNState returns the live managed-VPN split-tunnel state (whether the
+	// WireGuard tunnel is up, the mode, and the exit server) so the web can track
+	// which agent holds the single WG slot.
+	GetVPNState func() (active bool, mode, server string)
 	// OnDeleteFiles is called when the server requests file deletion from disk.
 	// It should delete the files and return the IDs of successfully deleted items.
 	OnDeleteFiles func(items []LibraryDeleteRequest) []int
@@ -155,6 +159,9 @@ func (sc *SyncClient) buildRequest() SyncRequest {
 	if sc.GetFreeSlots != nil {
 		req.FreeSlots = sc.GetFreeSlots()
 	}
+	if sc.GetVPNState != nil {
+		req.VPNActive, req.VPNMode, req.VPNServer = sc.GetVPNState()
+	}
 	// Flush confirmed deletions from previous cycle.
 	// Once flushed, remove IDs from deleteInFlight — the server will stop sending
 	// them after this sync, so deduplication protection is no longer needed.