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>

ui/README.md

@@ -132,6 +132,24 @@ make android         Capacitor + gradle                         Phase 32+
 make all             every target above
 ```
 
+## Local development
+
+For UI work against a real stack, the `tools/local-dev/` docker
+compose brings up postgres + redis + mailpit + backend + gateway in
+one command, and `ui/frontend/.env.development` is already wired to
+talk to it:
+
+```sh
+make -C tools/local-dev up        # build + start, wait for healthy
+pnpm -C ui/frontend dev           # Vite on the host
+# UI:     http://localhost:5173
+# Mailpit: http://localhost:8025
+```
+
+The stack accepts a fixed dev code (`123456`) in addition to the
+real Mailpit-delivered one. Full runbook in
+[`../tools/local-dev/README.md`](../tools/local-dev/README.md).
+
 ## Per-phase docs
 
 Topic docs live under `ui/docs/` and are added per phase as they're