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/frontend/src/proto/galaxy/fbs/lobby/error-response.ts

@@ -0,0 +1,77 @@
+// automatically generated by the FlatBuffers compiler, do not modify
+
+/* eslint-disable @typescript-eslint/no-unused-vars, @typescript-eslint/no-explicit-any, @typescript-eslint/no-non-null-assertion */
+
+import * as flatbuffers from 'flatbuffers';
+
+import { ErrorBody, ErrorBodyT } from './error-body.js';
+
+
+export class ErrorResponse implements flatbuffers.IUnpackableObject<ErrorResponseT> {
+  bb: flatbuffers.ByteBuffer|null = null;
+  bb_pos = 0;
+  __init(i:number, bb:flatbuffers.ByteBuffer):ErrorResponse {
+  this.bb_pos = i;
+  this.bb = bb;
+  return this;
+}
+
+static getRootAsErrorResponse(bb:flatbuffers.ByteBuffer, obj?:ErrorResponse):ErrorResponse {
+  return (obj || new ErrorResponse()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+static getSizePrefixedRootAsErrorResponse(bb:flatbuffers.ByteBuffer, obj?:ErrorResponse):ErrorResponse {
+  bb.setPosition(bb.position() + flatbuffers.SIZE_PREFIX_LENGTH);
+  return (obj || new ErrorResponse()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+error(obj?:ErrorBody):ErrorBody|null {
+  const offset = this.bb!.__offset(this.bb_pos, 4);
+  return offset ? (obj || new ErrorBody()).__init(this.bb!.__indirect(this.bb_pos + offset), this.bb!) : null;
+}
+
+static startErrorResponse(builder:flatbuffers.Builder) {
+  builder.startObject(1);
+}
+
+static addError(builder:flatbuffers.Builder, errorOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(0, errorOffset, 0);
+}
+
+static endErrorResponse(builder:flatbuffers.Builder):flatbuffers.Offset {
+  const offset = builder.endObject();
+  return offset;
+}
+
+static createErrorResponse(builder:flatbuffers.Builder, errorOffset:flatbuffers.Offset):flatbuffers.Offset {
+  ErrorResponse.startErrorResponse(builder);
+  ErrorResponse.addError(builder, errorOffset);
+  return ErrorResponse.endErrorResponse(builder);
+}
+
+unpack(): ErrorResponseT {
+  return new ErrorResponseT(
+    (this.error() !== null ? this.error()!.unpack() : null)
+  );
+}
+
+
+unpackTo(_o: ErrorResponseT): void {
+  _o.error = (this.error() !== null ? this.error()!.unpack() : null);
+}
+}
+
+export class ErrorResponseT implements flatbuffers.IGeneratedObject {
+constructor(
+  public error: ErrorBodyT|null = null
+){}
+
+
+pack(builder:flatbuffers.Builder): flatbuffers.Offset {
+  const error = (this.error !== null ? this.error!.pack(builder) : 0);
+
+  return ErrorResponse.createErrorResponse(builder,
+    error
+  );
+}
+}