tools/local-dev: docker-compose stack for UI development

Adds tools/local-dev/ with postgres + redis + mailpit + backend +
gateway plus a Make wrapper, so `make -C tools/local-dev up` brings
the full authenticated stack online and `pnpm -C ui/frontend dev`
talks to it directly. The committed `.env.development` already
points at the stack and pins the matching gateway response public
key from the dev keypair under tools/local-dev/keys/.

The backend ships a new opt-in env, BACKEND_AUTH_DEV_FIXED_CODE
(`tools/local-dev/.env` defaults it to 123456). When set,
ConfirmEmailCode accepts that literal in addition to the real
bcrypt-verified code; SendEmailCode still queues a real email so
Mailpit captures the issued code at http://localhost:8025/, and
both paths coexist. The override is rejected as non-six-digit by
config validation and emits a loud warning at backend startup.

The local-dev Dockerfiles mirror backend/Dockerfile and
gateway/Dockerfile but switch the runtime stage to alpine so
docker-compose healthchecks can wget /healthz; the gateway
Dockerfile additionally copies ui/core/ into the build context
because gateway/go.mod's `replace galaxy/core => ../ui/core` is
required to compile the gateway main.

Smoke tested:
- `make -C tools/local-dev up` boots all five services to healthy.
- send-email-code + confirm-email-code with code=123456 returns a
  device_session_id; a real code in Mailpit also redeems
  successfully.
- `pnpm test` 14/14, `pnpm exec playwright test` 44/44.
- `go test ./backend/internal/config/...` green.

Docs: tools/local-dev/README.md, tools/local-dev/keys/README.md,
new "Local development stack" section in ui/docs/testing.md, and a
short pointer in ui/README.md.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/internal/config/config_test.go

@@ -77,6 +77,40 @@ func TestValidateRejectsUnknownTracesExporter(t *testing.T) {
 	}
 }
 
+func TestLoadFromEnvAcceptsDevFixedCode(t *testing.T) {
+	env := validEnv()
+	env["BACKEND_AUTH_DEV_FIXED_CODE"] = "123456"
+	setEnv(t, env)
+
+	cfg, err := LoadFromEnv()
+	if err != nil {
+		t.Fatalf("LoadFromEnv returned error: %v", err)
+	}
+	if cfg.Auth.DevFixedCode != "123456" {
+		t.Fatalf("Auth.DevFixedCode = %q, want \"123456\"", cfg.Auth.DevFixedCode)
+	}
+}
+
+func TestValidateRejectsDevFixedCodeWrongLength(t *testing.T) {
+	env := validEnv()
+	env["BACKEND_AUTH_DEV_FIXED_CODE"] = "12345"
+	setEnv(t, env)
+
+	if _, err := LoadFromEnv(); err == nil || !strings.Contains(err.Error(), "BACKEND_AUTH_DEV_FIXED_CODE") {
+		t.Fatalf("expected DEV fixed-code length error, got %v", err)
+	}
+}
+
+func TestValidateRejectsDevFixedCodeNonDecimal(t *testing.T) {
+	env := validEnv()
+	env["BACKEND_AUTH_DEV_FIXED_CODE"] = "abcdef"
+	setEnv(t, env)
+
+	if _, err := LoadFromEnv(); err == nil || !strings.Contains(err.Error(), "BACKEND_AUTH_DEV_FIXED_CODE") {
+		t.Fatalf("expected DEV fixed-code decimal error, got %v", err)
+	}
+}
+
 func TestValidateRejectsPrometheusWithoutAddr(t *testing.T) {
 	cfg := DefaultConfig()
 	cfg.Postgres.DSN = "postgres://x:y@127.0.0.1/galaxy"