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/user/error-body.ts

@@ -0,0 +1,95 @@
+// 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';
+
+
+
+export class ErrorBody implements flatbuffers.IUnpackableObject<ErrorBodyT> {
+  bb: flatbuffers.ByteBuffer|null = null;
+  bb_pos = 0;
+  __init(i:number, bb:flatbuffers.ByteBuffer):ErrorBody {
+  this.bb_pos = i;
+  this.bb = bb;
+  return this;
+}
+
+static getRootAsErrorBody(bb:flatbuffers.ByteBuffer, obj?:ErrorBody):ErrorBody {
+  return (obj || new ErrorBody()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+static getSizePrefixedRootAsErrorBody(bb:flatbuffers.ByteBuffer, obj?:ErrorBody):ErrorBody {
+  bb.setPosition(bb.position() + flatbuffers.SIZE_PREFIX_LENGTH);
+  return (obj || new ErrorBody()).__init(bb.readInt32(bb.position()) + bb.position(), bb);
+}
+
+code():string|null
+code(optionalEncoding:flatbuffers.Encoding):string|Uint8Array|null
+code(optionalEncoding?:any):string|Uint8Array|null {
+  const offset = this.bb!.__offset(this.bb_pos, 4);
+  return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
+}
+
+message():string|null
+message(optionalEncoding:flatbuffers.Encoding):string|Uint8Array|null
+message(optionalEncoding?:any):string|Uint8Array|null {
+  const offset = this.bb!.__offset(this.bb_pos, 6);
+  return offset ? this.bb!.__string(this.bb_pos + offset, optionalEncoding) : null;
+}
+
+static startErrorBody(builder:flatbuffers.Builder) {
+  builder.startObject(2);
+}
+
+static addCode(builder:flatbuffers.Builder, codeOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(0, codeOffset, 0);
+}
+
+static addMessage(builder:flatbuffers.Builder, messageOffset:flatbuffers.Offset) {
+  builder.addFieldOffset(1, messageOffset, 0);
+}
+
+static endErrorBody(builder:flatbuffers.Builder):flatbuffers.Offset {
+  const offset = builder.endObject();
+  return offset;
+}
+
+static createErrorBody(builder:flatbuffers.Builder, codeOffset:flatbuffers.Offset, messageOffset:flatbuffers.Offset):flatbuffers.Offset {
+  ErrorBody.startErrorBody(builder);
+  ErrorBody.addCode(builder, codeOffset);
+  ErrorBody.addMessage(builder, messageOffset);
+  return ErrorBody.endErrorBody(builder);
+}
+
+unpack(): ErrorBodyT {
+  return new ErrorBodyT(
+    this.code(),
+    this.message()
+  );
+}
+
+
+unpackTo(_o: ErrorBodyT): void {
+  _o.code = this.code();
+  _o.message = this.message();
+}
+}
+
+export class ErrorBodyT implements flatbuffers.IGeneratedObject {
+constructor(
+  public code: string|Uint8Array|null = null,
+  public message: string|Uint8Array|null = null
+){}
+
+
+pack(builder:flatbuffers.Builder): flatbuffers.Offset {
+  const code = (this.code !== null ? builder.createString(this.code!) : 0);
+  const message = (this.message !== null ? builder.createString(this.message!) : 0);
+
+  return ErrorBody.createErrorBody(builder,
+    code,
+    message
+  );
+}
+}