feat(rooms): real server-side private games with friend invites (no bot swap)

Private rooms were 100% client-simulated (the "friend" auto-accepted then bots
filled invited seats). Now they're server-authoritative over SignalR:

Server (GameManager.PrivateRooms + GameHub):
- Room registry with create/invite/accept/decline/addBot/clearSeat/start/leave.
- Invite pushes a `roomInvite` to that user (Clients.User); the seat stays
  "invited" (a pending guest with their real profile, resolved server-side) — it
  is NEVER replaced by a bot.
- StartPrivate refuses while any invite is pending; only EMPTY seats fill with
  bots. Then it spins up a live GameRoom and matchFound → both devices enter.
- Host leave / disconnect closes the room (roomClosed); members free their seat.

Client:
- signalr-service implements the room methods over the hub (+ room/roomInvite/
  roomClosed events, room mapping, onRoomInvite); mock keeps offline no-ops.
- online-store accept/declineInvite; RoomScreen blocks "Start" while an invite
  is pending and auto-enters the live game on matchFound (host + friend).
- New global InviteModal (accept/decline) + i18n (invite.*, room.waitAccept).

Addresses: (1) no bot replacement, (2) game waits for acceptance, (3) invited
friend shown as a pending guest with their name/avatar.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/lib/online-store.ts

@@ -32,6 +32,8 @@ interface OnlineStore {
   clearSeat: (seat: 1 | 2 | 3) => Promise<void>;
   startRoom: () => Promise<void>;
   leaveRoom: () => Promise<void>;
+  acceptInvite: () => Promise<void>;
+  declineInvite: () => Promise<void>;
 
   startMatchmaking: (opts: MatchmakingOptions) => Promise<void>;
   cancelMatchmaking: () => Promise<void>;
@@ -133,6 +135,15 @@ export const useOnlineStore = create<OnlineStore>((set, get) => ({
     }
     set({ room: null });
   },
+  acceptInvite: async () => {
+    const svc = getService();
+    if (roomUnsub) roomUnsub();
+    roomUnsub = svc.onRoom((r) => set({ room: { ...r } })); // subscribe first so we catch the room push
+    await svc.acceptInvite();
+  },
+  declineInvite: async () => {
+    await getService().declineInvite();
+  },
 
   startMatchmaking: async (opts) => {
     const svc = getService();