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/lib/useIsDesktop.ts

@@ -0,0 +1,20 @@
+import { useEffect, useState } from 'react';
+
+/** True on viewports >= 1024px — drives the desktop GameTable layout
+ *  (score sidebar, larger oval, bigger cards). */
+export function useIsDesktop(): boolean {
+  const query = '(min-width: 1024px)';
+  const [isDesktop, setIsDesktop] = useState(
+    () => typeof window !== 'undefined' && window.matchMedia(query).matches,
+  );
+
+  useEffect(() => {
+    const mql = window.matchMedia(query);
+    const handler = (e: MediaQueryListEvent) => setIsDesktop(e.matches);
+    mql.addEventListener('change', handler);
+    setIsDesktop(mql.matches);
+    return () => mql.removeEventListener('change', handler);
+  }, []);
+
+  return isDesktop;
+}