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/auth/auth_e2e_test.go

@@ -185,6 +185,35 @@ func authConfig() config.AuthConfig {
 	}
 }
 
+// buildServiceWithConfig wires every dependency around db using cfg as
+// the auth configuration. Returns only the service — assertions on the
+// dev-mode override path do not inspect the recording fakes.
+func buildServiceWithConfig(t *testing.T, db *sql.DB, cfg config.AuthConfig) *auth.Service {
+	t.Helper()
+	store := auth.NewStore(db)
+	cache := auth.NewCache()
+	if err := cache.Warm(context.Background(), store); err != nil {
+		t.Fatalf("warm cache: %v", err)
+	}
+	userStore := user.NewStore(db)
+	userSvc := user.NewService(user.Deps{
+		Store:              userStore,
+		Cache:              user.NewCache(),
+		UserNameMaxRetries: 10,
+		Now:                time.Now,
+	})
+	return auth.NewService(auth.Deps{
+		Store:  store,
+		Cache:  cache,
+		User:   userSvc,
+		Geo:    newStubGeo(),
+		Mail:   newRecordingMailer(),
+		Push:   newRecordingPush(),
+		Config: cfg,
+		Now:    time.Now,
+	})
+}
+
 // buildService wires every dependency around db and returns the service
 // plus the recording fakes for assertions.
 func buildService(t *testing.T, db *sql.DB) (*auth.Service, *recordingMailer, *recordingPush, *stubGeo) {
@@ -412,6 +441,55 @@ func TestSendEmailCodeThrottleReusesChallenge(t *testing.T) {
 	}
 }
 
+func TestConfirmEmailCodeDevFixedCodeBypass(t *testing.T) {
+	db := startPostgres(t)
+	cfg := authConfig()
+	cfg.DevFixedCode = "999999"
+	svc := buildServiceWithConfig(t, db, cfg)
+	ctx := context.Background()
+
+	id, err := svc.SendEmailCode(ctx, "dev-bypass@example.test", "en", "", "")
+	if err != nil {
+		t.Fatalf("send: %v", err)
+	}
+
+	session, err := svc.ConfirmEmailCode(ctx, auth.ConfirmInputs{
+		ChallengeID:     id,
+		Code:            "999999",
+		ClientPublicKey: randomKey(t),
+		TimeZone:        "UTC",
+	})
+	if err != nil {
+		t.Fatalf("ConfirmEmailCode with dev fixed code: %v", err)
+	}
+	if session.DeviceSessionID == uuid.Nil {
+		t.Fatalf("dev fixed code did not produce a session")
+	}
+}
+
+func TestConfirmEmailCodeDevFixedCodeStillRejectsWrong(t *testing.T) {
+	db := startPostgres(t)
+	cfg := authConfig()
+	cfg.DevFixedCode = "999999"
+	svc := buildServiceWithConfig(t, db, cfg)
+	ctx := context.Background()
+
+	id, err := svc.SendEmailCode(ctx, "dev-bypass-wrong@example.test", "en", "", "")
+	if err != nil {
+		t.Fatalf("send: %v", err)
+	}
+
+	_, err = svc.ConfirmEmailCode(ctx, auth.ConfirmInputs{
+		ChallengeID:     id,
+		Code:            "111111",
+		ClientPublicKey: randomKey(t),
+		TimeZone:        "UTC",
+	})
+	if !errors.Is(err, auth.ErrCodeMismatch) {
+		t.Fatalf("ConfirmEmailCode with neither real nor dev code = %v, want ErrCodeMismatch", err)
+	}
+}
+
 func TestConfirmEmailCodeWrongCode(t *testing.T) {
 	db := startPostgres(t)
 	svc, mailer, _, _ := buildService(t, db)