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/invite/invite.go

@@ -0,0 +1,54 @@
+// Package invite encodes and decodes waste invite strings.
+// An invite carries the anchor URL and network name needed to join a network.
+// Format: "waste:<url-safe-base64(json)>"
+package invite
+
+import (
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"strings"
+)
+
+const prefix = "waste:"
+
+// Invite holds the information needed to join a network.
+type Invite struct {
+	Anchor  string `json:"anchor"`  // WebSocket anchor URL
+	Network string `json:"network"` // plaintext network name
+}
+
+// Encode returns a waste: invite string for the given anchor URL and network name.
+func Encode(anchor, network string) (string, error) {
+	if anchor == "" {
+		return "", fmt.Errorf("anchor URL is required")
+	}
+	if network == "" {
+		return "", fmt.Errorf("network name is required")
+	}
+	b, err := json.Marshal(Invite{Anchor: anchor, Network: network})
+	if err != nil {
+		return "", err
+	}
+	return prefix + base64.URLEncoding.EncodeToString(b), nil
+}
+
+// Decode parses a waste: invite string and returns the Invite.
+func Decode(s string) (Invite, error) {
+	s = strings.TrimSpace(s)
+	if !strings.HasPrefix(s, prefix) {
+		return Invite{}, fmt.Errorf("not a waste invite (expected 'waste:' prefix)")
+	}
+	b, err := base64.URLEncoding.DecodeString(strings.TrimPrefix(s, prefix))
+	if err != nil {
+		return Invite{}, fmt.Errorf("invalid invite: %w", err)
+	}
+	var inv Invite
+	if err := json.Unmarshal(b, &inv); err != nil {
+		return Invite{}, fmt.Errorf("invalid invite payload: %w", err)
+	}
+	if inv.Anchor == "" || inv.Network == "" {
+		return Invite{}, fmt.Errorf("invite is missing anchor or network name")
+	}
+	return inv, nil
+}

internal/invite/invite_test.go

@@ -0,0 +1,38 @@
+package invite
+
+import "testing"
+
+func TestRoundTrip(t *testing.T) {
+	s, err := Encode("ws://my-vps:17339/ws", "friends")
+	if err != nil {
+		t.Fatalf("Encode: %v", err)
+	}
+	if len(s) < 7 || s[:6] != "waste:" {
+		t.Fatalf("expected waste: prefix, got %q", s)
+	}
+
+	inv, err := Decode(s)
+	if err != nil {
+		t.Fatalf("Decode: %v", err)
+	}
+	if inv.Anchor != "ws://my-vps:17339/ws" {
+		t.Fatalf("anchor = %q", inv.Anchor)
+	}
+	if inv.Network != "friends" {
+		t.Fatalf("network = %q", inv.Network)
+	}
+}
+
+func TestDecodeInvalid(t *testing.T) {
+	cases := []string{
+		"",
+		"notaninvite",
+		"waste:!!!",
+		"waste:" + "eyJhbmNob3IiOiIifQ==", // missing network
+	}
+	for _, c := range cases {
+		if _, err := Decode(c); err == nil {
+			t.Errorf("Decode(%q) expected error, got nil", c)
+		}
+	}
+}