Add invite strings (waste: URI) for peer onboarding

- internal/invite: Encode/Decode waste:<base64json{anchor,network}>
- proto: CmdGenerateInvite + EvtInviteGenerated + InviteString field
- ipc: track active network name; handle generate_invite (returns invite
  string when joined to a network, errors if not joined or no anchor)
- daemon: --join <invite> flag — decodes anchor URL + network name,
  sets anchor and auto-joins on startup
- tui: --join <invite> flag — extracts network name, skips -network
  requirement; ctrl+i generates invite and shows it full-screen;
  Esc dismisses the invite overlay
- FUTURE.md: document multi-network derived-identity design

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/proto/proto.go

@@ -185,7 +185,8 @@ const (
 	CmdJoinNetwork  IpcMsgType = "join_network"  // fields: network_name (plaintext)
 	CmdLeaveNetwork IpcMsgType = "leave_network"
 	CmdGetState     IpcMsgType = "get_state"
-	CmdSendFile     IpcMsgType = "send_file"
+	CmdSendFile      IpcMsgType = "send_file"
+	CmdGenerateInvite IpcMsgType = "generate_invite"
 
 	// Events (daemon → UI)
 	EvtMessageReceived  IpcMsgType = "message_received"
@@ -196,6 +197,7 @@ const (
 	EvtFileProgress     IpcMsgType = "file_progress"
 	EvtStateSnapshot    IpcMsgType = "state_snapshot"
 	EvtError            IpcMsgType = "error"
+	EvtInviteGenerated  IpcMsgType = "invite_generated"
 )
 
 // IpcMessage covers both commands and events.
@@ -226,4 +228,5 @@ type IpcMessage struct {
 	ConnectedPeers []PeerInfo   `json:"connected_peers,omitempty"`
 	Rooms          []string     `json:"rooms,omitempty"`
 	ErrorMessage   string       `json:"error_message,omitempty"`
+	InviteString   string       `json:"invite,omitempty"`
 }