Apply velvet-table redesign, fix game lifecycle and history bugs

Frontend:
- Dark green/gold "velvet table" visual redesign across the whole app
  (Auth, Lobby, GameList, GameTable, History, GameOver, modals), with
  Playfair Display/DM Sans typography and a centralized Tailwind palette.
- Desktop game table fit-scales to fill the window; mobile gets
  overlapping hand/trick layouts and larger touch-friendly cards.
- Standings sidebar now groups completed rounds by series with a
  per-series subtotal row, struck-through tips on missed bids.
- History page rewritten into a scoreboard-style detail view (player
  totals beside names, series grouped 2-up on desktop / stacked on
  mobile) and gained game names, completed/abandoned status, and a
  button to reopen a prematurely-ended game back into the lobby.

Backend:
- Fix started games being deleted from memory (and vanishing from
  everyone's lobby) when all players disconnect; only `end_game` tears
  down a started game now.
- Fix a crash writing a timezone-aware datetime into the naive
  `ended_at` Postgres column.
- Add `reopen_game`/`restore_game` to un-end a prematurely-ended game
  from history and resume it from the lobby.
- Let any seated player end an abandoned game once the host is
  offline, not just the host, so the game isn't stuck forever.
- Expose SERIES_PER_GAME/ROUNDS_PER_SERIES as named constants on the
  engine so the persistence layer derives game-completion rules from
  bridzik.py instead of re-encoding them.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/FaceDownCards.tsx

@@ -0,0 +1,43 @@
+interface Props {
+  /** Exact number of cards the player still holds. */
+  count: number;
+  /** Row (top player) or column (side players). */
+  direction: 'row' | 'col';
+  desktop?: boolean;
+}
+
+/** Overlapping fan of face-down cards next to an opponent's circle — one card
+ *  per card still in their hand, so the stack shrinks as they play. */
+export default function FaceDownCards({ count, direction, desktop = false }: Props) {
+  const n = Math.max(0, Math.min(count, 8));
+  if (n === 0) return null;
+
+  const row = direction === 'row';
+  const w = desktop ? 40 : 25;
+  const h = desktop ? 56 : 36;
+  const overlap = row ? Math.round(w * 0.45) : Math.round(h * 0.5);
+
+  return (
+    <div className="flex" style={{ flexDirection: row ? 'row' : 'column' }}>
+      {Array.from({ length: n }).map((_, i) => (
+        <div
+          key={i}
+          style={{
+            width: w,
+            height: h,
+            marginLeft: row && i > 0 ? -overlap : 0,
+            marginTop: !row && i > 0 ? -overlap : 0,
+            zIndex: i,
+            background:
+              i % 2 === 0
+                ? 'linear-gradient(150deg,#1d4a28,#0e2818)'
+                : 'linear-gradient(150deg,#1b4424,#0d2616)',
+            borderRadius: 3,
+            border: '1px solid rgba(201,168,76,.18)',
+            boxShadow: '0 2px 6px rgba(0,0,0,.5)',
+          }}
+        />
+      ))}
+    </div>
+  );
+}