Add hand-end scoring summary screen with acknowledgement

After each hand of an unfinished match the game now pauses in a new
hand_end phase instead of dealing immediately:

- engine: hand_points gains an 'award' map (which team won each category),
  _end_hand stops at hand_end with a deadline, new acknowledge_hand deals
  the next hand once all four players have acked; plays are rejected while
  the summary is up
- state: acked seats, hand_end_deadline and hand_ack_timeout are persisted
  and exposed in the personalized view (also on the finished state, so the
  final hand is explained before the result)
- ws: new {"action": "ack"}; a per-hand timer force-deals the next hand
  after HAND_ACK_TIMEOUT_SECONDS (new env var, default 30s) so an away
  player cannot stall the match
- web: modal explaining each category in plain language with icons (card
  images for denara/settebello/primiera), team-coloured rows, running
  totals with progress bars, an 'Understood — next hand' button that turns
  into 'Waiting for …' plus an auto-continue countdown; the final screen
  shows the last hand's breakdown too

Verified in the browser against the compose stack: hand played to
completion, summary rendered (including a carte tie), ack from all four
players dealt the next hand live, and the auto-continue path fired when
nobody acked. 60 backend tests + mypy + cargo tests green.

server/README.md

@@ -50,6 +50,7 @@ All configuration comes from environment variables (see `.env.example`):
 | `OIDC_CLIENT_SECRET` | unset | OIDC client secret |
 | `OIDC_REDIRECT_URI` | `http://localhost:8080/auth/callback` | Login callback URL |
 | `GAME_TTL_SECONDS` | `86400` | Sliding TTL of a live game in Redis |
+| `HAND_ACK_TIMEOUT_SECONDS` | `30` | Seconds the between-hands scoring summary waits for acknowledgements |
 | `APP_HOST` / `APP_PORT` | `0.0.0.0` / `8080` | Bind address |
 
 ## Data model
@@ -109,6 +110,7 @@ Client → server messages:
 ```json
 {"action": "play", "card": "07D", "capture": ["02D", "05C"]}
 {"action": "play", "card": "07D"}
+{"action": "ack"}
 {"action": "state"}
 ```
 
@@ -117,10 +119,23 @@ Client → server messages:
   settebello).
 - `capture` lists the table cards to take. When a capture is legal it is
   mandatory to provide one; when no capture exists it must be omitted.
+- `ack` acknowledges the hand-end scoring summary (see below). The next
+  hand is dealt once all four players have acknowledged, or automatically
+  after `HAND_ACK_TIMEOUT_SECONDS`.
 - `state` asks for a fresh snapshot.
 
 After every accepted move the new state is broadcast to all four players.
 
+### Hand-end summary
+
+When a hand finishes but the match continues, the game enters the
+`hand_end` phase instead of dealing immediately: the state carries
+`last_hand` (a full scoring breakdown with an `award` map naming the team
+that won each category), the `acknowledged` seats and a
+`hand_end_deadline`. The frontend renders this as a screen every player
+must dismiss. A play attempted in this phase is rejected with an
+`illegal_move` error.
+
 ## Rules implemented
 
 - 40-card Italian deck, ten cards per player, empty table at hand start.