feat: initial scaffold of uniweb — unibus web frontend (SPA + gateway)

Extracted from unibus v0.13.0: the chat SPA (web/, React+Mantine, per-user
BIP39 wallet) and the web gateway (cmd/webgw, REST+SSE) that acts as a bus
peer for the browser. Consumes unibus as a Go module via replace => ../unibus,
keeping its own replace fn-registry for the cybersecurity primitives.

go build/vet/test and pnpm build green in the new location.

cmd/webgw/identity.go

@@ -0,0 +1,98 @@
+package main
+
+import (
+	"encoding/base64"
+	"encoding/json"
+	"fmt"
+	"os"
+	"os/exec"
+
+	cs "fn-registry/functions/cybersecurity"
+)
+
+// identityJSON mirrors the on-disk / pass-stored identity format shared across
+// the unibus tooling: the four keypair halves, each std-base64. It is the SAME
+// shape the bus client persists (pkg/client identity file) and the operator's
+// `pass` entry unibus/operator-identity, so the web gateway loads the operator's
+// identity without a divergent serialization. Kept in lockstep with
+// unibus_admin/internal/admin/identity.go.
+type identityJSON struct {
+	SignPub  string `json:"sign_pub"`
+	SignPriv string `json:"sign_priv"`
+	KexPub   string `json:"kex_pub"`
+	KexPriv  string `json:"kex_priv"`
+}
+
+// decodeIdentity turns the JSON identity bytes into a cs.Identity. The private
+// halves stay only in memory; this never writes them anywhere.
+func decodeIdentity(raw []byte) (cs.Identity, error) {
+	var f identityJSON
+	if err := json.Unmarshal(raw, &f); err != nil {
+		return cs.Identity{}, fmt.Errorf("webgw: parse identity json: %w", err)
+	}
+	dec := base64.StdEncoding.DecodeString
+	signPub, err := dec(f.SignPub)
+	if err != nil {
+		return cs.Identity{}, fmt.Errorf("webgw: decode sign_pub: %w", err)
+	}
+	signPriv, err := dec(f.SignPriv)
+	if err != nil {
+		return cs.Identity{}, fmt.Errorf("webgw: decode sign_priv: %w", err)
+	}
+	kexPub, err := dec(f.KexPub)
+	if err != nil {
+		return cs.Identity{}, fmt.Errorf("webgw: decode kex_pub: %w", err)
+	}
+	kexPriv, err := dec(f.KexPriv)
+	if err != nil {
+		return cs.Identity{}, fmt.Errorf("webgw: decode kex_priv: %w", err)
+	}
+	if len(signPub) != 32 || len(signPriv) != 64 || len(kexPub) != 32 || len(kexPriv) != 32 {
+		return cs.Identity{}, fmt.Errorf("webgw: identity has wrong key sizes (sign_pub=%d sign_priv=%d kex_pub=%d kex_priv=%d)",
+			len(signPub), len(signPriv), len(kexPub), len(kexPriv))
+	}
+	return cs.Identity{SignPub: signPub, SignPriv: signPriv, KexPub: kexPub, KexPriv: kexPriv}, nil
+}
+
+// loadIdentityFromFile reads a 0600 identity JSON file (the same format the bus
+// client writes) and decodes it. Used on a deploy host where `pass` is not
+// available and the operator identity is delivered as a protected file.
+func loadIdentityFromFile(path string) (cs.Identity, error) {
+	raw, err := os.ReadFile(path)
+	if err != nil {
+		return cs.Identity{}, fmt.Errorf("webgw: read identity file %q: %w", path, err)
+	}
+	return decodeIdentity(raw)
+}
+
+// loadIdentityFromPass shells out to `pass show <entry>` and decodes the JSON
+// identity it returns. The secret is held only in memory; this process never
+// writes it to disk or argv. Used in local operator workflows where the GNU
+// password store holds unibus/operator-identity.
+func loadIdentityFromPass(entry string) (cs.Identity, error) {
+	out, err := exec.Command("pass", "show", entry).Output()
+	if err != nil {
+		return cs.Identity{}, fmt.Errorf("webgw: pass show %q: %w", entry, err)
+	}
+	return decodeIdentity(out)
+}
+
+// loadPassValue returns the first line of a `pass show <entry>` for non-identity
+// secrets (e.g. the unlock passphrase). Empty entry yields an empty string and
+// no error, so callers can treat "no pass entry configured" as "not set".
+func loadPassValue(entry string) (string, error) {
+	if entry == "" {
+		return "", nil
+	}
+	out, err := exec.Command("pass", "show", entry).Output()
+	if err != nil {
+		return "", fmt.Errorf("webgw: pass show %q: %w", entry, err)
+	}
+	s := string(out)
+	for i := 0; i < len(s); i++ {
+		if s[i] == '\n' || s[i] == '\r' {
+			return s[:i], nil
+		}
+	}
+	return s, nil
+}