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/docs/testing.md

@@ -83,6 +83,32 @@ go test -count=1 \
   ./pkg/storage/... ./pkg/transcoder/... ./pkg/util/...
 ```
 
+## Local development stack
+
+For UI work that needs a real authenticated stack (verifying the
+FlatBuffers wire end-to-end, exercising a real lobby flow, hitting
+real Mailpit), bring up `tools/local-dev/`:
+
+```sh
+make -C tools/local-dev up        # postgres + redis + mailpit + backend + gateway
+pnpm -C ui/frontend dev           # Vite on the host, talks to the stack
+```
+
+`ui/frontend/.env.development` already targets the stack
+(`http://localhost:8080`) and pins the matching response-signing
+public key from `tools/local-dev/keys/`. Per-developer overrides go
+into `.env.development.local` (gitignored).
+
+The stack honours `BACKEND_AUTH_DEV_FIXED_CODE` (default `123456` in
+`tools/local-dev/.env`) so the login form takes that literal in
+addition to the real Mailpit code; see
+[`../../tools/local-dev/README.md`](../../tools/local-dev/README.md)
+for the full runbook (regenerating the dev keypair, switching the
+mode off, troubleshooting common boot issues).
+
+The local-dev stack is independent from the local-ci stack below;
+they bind different ports and can run side by side.
+
 ## Local CI verification
 
 `tools/local-ci/` ships a self-contained Gitea + Actions runner via