phase 8: lobby UI + cross-stack lobby command catalog + TS FlatBuffers

- Extend pkg/model/lobby and pkg/schema/fbs/lobby.fbs with public-games
  list, my-applications/invites lists, game-create, application-submit,
  invite-redeem/decline. Mirror the matching transcoder pairs and Go
  fixture round-trip tests.
- Wire the seven new lobby message types through
  gateway/internal/backendclient/{routes,lobby_commands}.go with
  per-command REST helpers, JSON-tolerant decoding of backend wire
  shapes, and httptest-based unit coverage for success / 4xx / 5xx /
  503 across each command.
- Introduce TS-side FlatBuffers via the `flatbuffers` runtime dep, a
  `make fbs-ts` target driving flatc, and the generated bindings under
  ui/frontend/src/proto/galaxy/fbs. Phase 7's `user.account.get` decode
  now uses these bindings as well, closing the JSON.parse vs
  FlatBuffers gap that would have failed against a real local stack.
- Replace the placeholder lobby with five sections (my games, pending
  invitations, my applications, public games, create new game) and the
  /lobby/create form. Submit-application uses an inline race-name
  form on the public-game card; create-game keeps name / description /
  turn_schedule / enrollment_ends_at always visible and the rest under
  an Advanced toggle with TS-side defaults.
- Update lobby/+page.svelte to throw LobbyError on non-ok result codes;
  GalaxyClient.executeCommand now returns { resultCode, payloadBytes }.
- Vitest binding round-trips, lobby.ts wrapper unit tests, lobby-page
  + lobby-create component tests, Playwright lobby-flow.spec covering
  create / submit / accept across all four projects. Phase 7 e2e was
  migrated to the FlatBuffers fixtures and to click+fill against the
  Safari-autofill readonly inputs.
- Mark Phase 8 done in ui/PLAN.md, mirror the wire-format note into
  Phase 7, append the new lobby commands to gateway/README.md and
  docs/ARCHITECTURE.md, add ui/docs/lobby.md.

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

docs/ARCHITECTURE.md

@@ -145,6 +145,15 @@ because they cross domain boundaries:
   `X-User-ID`. Public games carry `owner_user_id IS NULL`; the partial
   index on `(owner_user_id) WHERE visibility = 'private'` keeps the
   private-owner lookup efficient.
+- **Authenticated lobby commands** flow through the gateway envelope
+  by `message_type`. The catalog is `lobby.my.games.list`,
+  `lobby.public.games.list`, `lobby.my.applications.list`,
+  `lobby.my.invites.list`, `lobby.game.create`,
+  `lobby.game.open-enrollment`, `lobby.application.submit`,
+  `lobby.invite.redeem`, and `lobby.invite.decline`. Each lands on a
+  REST handler under `/api/v1/user/lobby/*`; the gateway forces
+  visibility to `private` on `lobby.game.create` before forwarding,
+  matching the user-surface invariant above.
 
 | Package                          | Responsibility                                                                                                                                                          |
 | -------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------- |