api: centralize handler registration and move handlers to a separate file

Move all REST API endpoint registrations into a single file (pkg/virtualnetwork/mux.go) to make the full API surface visible more clearly. Previously, handlers were scattered across multiple files with a mux composition chain using StripPrefix.

The three API surfaces are now defined as explicit mux constructors:

• GatewayMux(): only forwarder endpoints (for gateway 192.168.127.1)
• ServicesMux(): everything except /connect (for -services)
• Mux(): full API including /connect (for -listen endpoints)

Service packages (forwarder, DNS, DHCP) no longer define HTTP handlers. Instead, they export business logic methods (List, Expose, Unexpose, Zones, AddZone, Leases) that mux.go calls from its handlers. This removes their dependency on net/http.

The VirtualNetwork struct now holds references to the DNS and DHCP servers, allowing direct access to their methods from the centralized mux handlers.

Move all HTTP handler implementations from mux.go to a new handlers.go file to improve code organization and readability.