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.go

@@ -76,9 +76,30 @@ func NewService(deps Deps) *Service {
 		// not a security primitive, so a constant key is acceptable.
 		copy(key, []byte("galaxy-backend-auth-fallback-key"))
 	}
+	if deps.Config.DevFixedCode != "" {
+		// Loud, repeated warning so a stray production deployment cannot
+		// claim the operator was unaware. The override is intended for
+		// `tools/local-dev/` and never reaches production binaries in
+		// normal operation.
+		deps.Logger.Warn("DEV-MODE: BACKEND_AUTH_DEV_FIXED_CODE is set; ConfirmEmailCode accepts the literal code in addition to the bcrypt-verified one. NEVER use in production.")
+	}
 	return &Service{deps: deps, emailHashKey: key}
 }
 
+// devFixedCodeMatches reports whether the dev-mode fixed-code override
+// is configured and the submitted code matches it verbatim. The
+// override is opt-in via `BACKEND_AUTH_DEV_FIXED_CODE`; production
+// deployments leave the field empty and devFixedCodeMatches always
+// returns false. See `tools/local-dev/README.md` for the full
+// rationale.
+func (s *Service) devFixedCodeMatches(code string) bool {
+	fixed := s.deps.Config.DevFixedCode
+	if fixed == "" {
+		return false
+	}
+	return code == fixed
+}
+
 // hashEmail returns a stable, hex-encoded HMAC-SHA256 prefix of email
 // suitable for use in structured logs. The key is per-process so the
 // same email maps to the same hash across log lines emitted by this