feat(nostr): nostr-based account registration and verification

Rebuilt the nostr registration feature cleanly on top of current ergo
mainline (origin/master), dropping the stale ergo reversions that were
entangled in the original squashed branch.

- New irc/nostr package: NIP-04/NIP-17 DMs, NIP-05 resolution, relay
  connect/auth, and identifier helpers.
- Registration/verification via nostr: parseCallback auto-detects nostr
  identifiers (NIP-05, npub, hex pubkey) and dispatches a verification DM.
- Nostr-based cloaked hostnames for accounts (opt-in via
  ip-cloaking.nostr-hostnames), applied at the account-cloak sites.
- Config: accounts.registration.nostr-verification and the
  nostr-hostnames cloak option, with matching default.yaml and help text.
- Adds github.com/nbd-wtf/go-nostr and vendored dependencies.

vendor/github.com/coder/websocket/internal/xsync/go.go

@@ -0,0 +1,26 @@
+package xsync
+
+import (
+	"fmt"
+	"runtime/debug"
+)
+
+// Go allows running a function in another goroutine
+// and waiting for its error.
+func Go(fn func() error) <-chan error {
+	errs := make(chan error, 1)
+	go func() {
+		defer func() {
+			r := recover()
+			if r != nil {
+				select {
+				case errs <- fmt.Errorf("panic in go fn: %v, %s", r, debug.Stack()):
+				default:
+				}
+			}
+		}()
+		errs <- fn()
+	}()
+
+	return errs
+}

vendor/github.com/coder/websocket/internal/xsync/int64.go

@@ -0,0 +1,23 @@
+package xsync
+
+import (
+	"sync/atomic"
+)
+
+// Int64 represents an atomic int64.
+type Int64 struct {
+	// We do not use atomic.Load/StoreInt64 since it does not
+	// work on 32 bit computers but we need 64 bit integers.
+	i atomic.Value
+}
+
+// Load loads the int64.
+func (v *Int64) Load() int64 {
+	i, _ := v.i.Load().(int64)
+	return i
+}
+
+// Store stores the int64.
+func (v *Int64) Store(i int64) {
+	v.i.Store(i)
+}