Let anyone with the game URL watch as a read-only spectator

Anonymous users and logged-in non-players can now open /game/<id> and
follow the match: they see the table, the score and each player's
captured cards and scope, but never a hand and never the join code.
Spectator actions are rejected with a forbidden error.

Backend (tavolo-platform / tavolo-scopone):
- /ws/games/{id} accepts anonymous and non-seated connections as
  spectators; only unknown games are refused. The state payload carries
  spectator: true and withholds join_code unless the viewer is seated.
- GET /api/games/{id} is public and returns the same personalized
  (player) or read-only (spectator) view.
- GameEngine.view_for/game_over_view take Optional[str] (None = spectator)
  and are documented to hide privileged information for non-seated
  viewers.

Frontend (Sycamore):
- Parse the spectator flag and render a spectator table: card backs plus
  captured/scope for every seat, no clickable hand, "Spectating" badge.
- A pre-start spectator sees a "game not started yet" waiting room with
  the watch link and no join code.
- "Copy watch link" buttons in the waiting room and the game HUD.
- The hand-end summary carries no acknowledge button for spectators and
  the game-over overlay hides "My matches".

Tests and docs updated.

server/packages/tavolo-platform/tests/helpers.py

@@ -138,7 +138,7 @@ class DummyEngine(GameEngine):
             state["finished"] = True
             state["winner"] = user_sub
 
-    def view_for(self, session: GameSession, user_sub: str) -> Dict[str, Any]:
+    def view_for(self, session: GameSession, user_sub: Optional[str]) -> Dict[str, Any]:
         state = session.state
         return {
             "started": state["started"],
@@ -146,7 +146,7 @@ class DummyEngine(GameEngine):
             "winner": state["winner"],
             "plays": len(state["plays"]),
             "target": state["target"],
-            "viewer_seated": session.seated(user_sub),
+            "viewer_seated": user_sub is not None and session.seated(user_sub),
         }
 
     def lobby_view(self, session: GameSession) -> Dict[str, Any]: