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>

pkg/schema/fbs/lobby.fbs

@@ -4,7 +4,8 @@
 // from the verified session and forwards it as `X-User-Id` to lobby.
 namespace lobby;
 
-// GameSummary stores one game record returned by `lobby.my.games.list`.
+// GameSummary stores one game record returned by the lobby list
+// endpoints. owner_user_id is empty for public games (no human owner).
 // The shape matches `lobby/openapi.yaml` `MyGameSummary`.
 table GameSummary {
   game_id:string;
@@ -31,6 +32,71 @@ table MyGamesListResponse {
   items:[GameSummary];
 }
 
+// PublicGamesListRequest stores the paginated read request for joinable
+// public games. Page numbers start at 1; backend caps page_size.
+table PublicGamesListRequest {
+  page:int32;
+  page_size:int32;
+}
+
+// PublicGamesListResponse stores one page of public games together with
+// the pagination metadata.
+table PublicGamesListResponse {
+  items:[GameSummary];
+  page:int32;
+  page_size:int32;
+  total:int32;
+}
+
+// ApplicationSummary stores one application record. decided_at_ms is 0
+// while the application is pending.
+table ApplicationSummary {
+  application_id:string;
+  game_id:string;
+  applicant_user_id:string;
+  race_name:string;
+  status:string;
+  created_at_ms:int64;
+  decided_at_ms:int64;
+}
+
+// MyApplicationsListRequest stores the authenticated read request for
+// the caller's applications. Empty body.
+table MyApplicationsListRequest {
+}
+
+// MyApplicationsListResponse stores the caller's applications in the
+// order the backend returns them.
+table MyApplicationsListResponse {
+  items:[ApplicationSummary];
+}
+
+// InviteSummary stores one invite record. invited_user_id is empty for
+// code-based invites; code is empty for user-bound invites;
+// decided_at_ms is 0 while the invite is still pending.
+table InviteSummary {
+  invite_id:string;
+  game_id:string;
+  inviter_user_id:string;
+  invited_user_id:string;
+  code:string;
+  race_name:string;
+  status:string;
+  created_at_ms:int64;
+  expires_at_ms:int64;
+  decided_at_ms:int64;
+}
+
+// MyInvitesListRequest stores the authenticated read request for the
+// caller's invites. Empty body.
+table MyInvitesListRequest {
+}
+
+// MyInvitesListResponse stores the caller's invite list.
+table MyInvitesListResponse {
+  items:[InviteSummary];
+}
+
 // OpenEnrollmentRequest stores the owner-only command that transitions
 // a game from `draft` to `enrollment_open`.
 table OpenEnrollmentRequest {
@@ -43,6 +109,65 @@ table OpenEnrollmentResponse {
   status:string;
 }
 
+// GameCreateRequest stores the create-game command. Visibility is
+// always `private` for the user surface; the gateway rejects any other
+// value before forwarding to backend.
+table GameCreateRequest {
+  game_name:string;
+  description:string;
+  min_players:int32;
+  max_players:int32;
+  start_gap_hours:int32;
+  start_gap_players:int32;
+  enrollment_ends_at_ms:int64;
+  turn_schedule:string;
+  target_engine_version:string;
+}
+
+// GameCreateResponse wraps the freshly created game projected onto the
+// shared GameSummary shape.
+table GameCreateResponse {
+  game:GameSummary;
+}
+
+// ApplicationSubmitRequest stores the submit-application command for a
+// public game in `enrollment_open`.
+table ApplicationSubmitRequest {
+  game_id:string;
+  race_name:string;
+}
+
+// ApplicationSubmitResponse wraps the freshly created application
+// record.
+table ApplicationSubmitResponse {
+  application:ApplicationSummary;
+}
+
+// InviteRedeemRequest accepts a pending invite and creates the
+// corresponding membership.
+table InviteRedeemRequest {
+  game_id:string;
+  invite_id:string;
+}
+
+// InviteRedeemResponse wraps the updated invite record (status
+// transitioned to `accepted`).
+table InviteRedeemResponse {
+  invite:InviteSummary;
+}
+
+// InviteDeclineRequest terminally declines a pending invite.
+table InviteDeclineRequest {
+  game_id:string;
+  invite_id:string;
+}
+
+// InviteDeclineResponse wraps the updated invite record (status
+// transitioned to `declined`).
+table InviteDeclineResponse {
+  invite:InviteSummary;
+}
+
 // ErrorBody stores the canonical lobby error envelope code/message
 // pair.
 table ErrorBody {