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>

ui/Makefile

@@ -1,15 +1,18 @@
-.PHONY: help web wasm ts-protos gomobile desktop-mac desktop-win desktop-linux ios android all
+.PHONY: help web wasm ts-protos fbs-ts gomobile desktop-mac desktop-win desktop-linux ios android all
 
 .DEFAULT_GOAL := help
 
 WASM_OUT    := frontend/static/core.wasm
 WASM_EXEC   := frontend/static/wasm_exec.js
 TINYGO_ROOT := $(shell tinygo env TINYGOROOT 2>/dev/null)
+FBS_OUT     := frontend/src/proto/galaxy/fbs
+FBS_INPUTS  := ../pkg/schema/fbs/lobby.fbs ../pkg/schema/fbs/user.fbs
 
 help:
 	@echo "ui targets:"
 	@echo "  wasm           TinyGo build of ui/core to core.wasm + wasm_exec.js shim (Phase 5)"
 	@echo "  ts-protos      Connect-ES + Protobuf-ES generation from gateway/proto (Phase 5)"
+	@echo "  fbs-ts         FlatBuffers TS generation from pkg/schema/fbs/*.fbs (Phase 8)"
 	@echo "  web            Vite production build (Phase 5+)"
 	@echo "  gomobile       gomobile bind for iOS .framework + Android .aar (Phase 32+)"
 	@echo "  desktop-mac    Wails build for darwin/{arm64,amd64} (Phase 31)"
@@ -30,6 +33,11 @@ ts-protos:
 	@test -x frontend/node_modules/.bin/protoc-gen-es || { echo "protoc-gen-es not installed; run 'pnpm install' inside ui/frontend"; exit 1; }
 	buf generate ../gateway --template buf.gen.yaml --include-imports
 
+fbs-ts:
+	@command -v flatc >/dev/null || { echo "flatc not found; install via 'brew install flatbuffers' (macOS) or 'apt-get install -y flatbuffers-compiler' (Linux)"; exit 1; }
+	mkdir -p $(FBS_OUT)
+	flatc --ts --gen-object-api -o $(FBS_OUT) $(FBS_INPUTS)
+
 web gomobile desktop-mac desktop-win desktop-linux ios android all:
 	@echo "TODO: implement '$@' (placeholder, see ui/PLAN.md)"
 	@exit 1