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/RulesModal.tsx

@@ -9,17 +9,17 @@ export default function RulesModal({ onClose }: Props) {
       onClick={onClose}
     >
       <div
-        className="relative bg-slate-900 rounded-2xl w-full max-w-lg my-6 p-6 text-sm leading-relaxed"
+        className="relative bg-header border border-[#142018] rounded-2xl w-full max-w-lg my-6 p-6 text-sm leading-relaxed text-green-score shadow-[0_28px_88px_rgba(0,0,0,.65)]"
         onClick={(e) => e.stopPropagation()}
       >
         <button
           onClick={onClose}
-          className="absolute top-4 right-4 text-gray-400 hover:text-white text-xl leading-none"
+          className="absolute top-4 right-4 text-green-dim hover:text-gold text-xl leading-none"
         >
           ✕
         </button>
 
-        <h1 className="text-xl font-bold mb-4">Pravidlá hry Bridžik</h1>
+        <h1 className="font-serif text-2xl text-gold mb-4">Pravidlá hry Bridžik</h1>
 
         <Section title="Karty">
           <p>Hrá sa s <b>32-kartovým balíčkom</b> sedmových (slovenských/nemeckých) kariet.</p>
@@ -37,7 +37,7 @@ export default function RulesModal({ onClose }: Props) {
         <Section title="Priebeh kola">
           <p className="font-semibold">1. Tipovanie</p>
           <p className="mt-1">Každý hráč tipuje, koľko kopiek v kole získa (0 až počet kopiek).</p>
-          <p className="mt-1 text-yellow-300">Pravidlo bridžika: súčet tipov nesmie presne rovnať počtu kopiek v kole — posledný tipujúci nemôže zadať tip, ktorý by toto spôsobil.</p>
+          <p className="mt-1 text-gold-dim">Pravidlo bridžika: súčet tipov nesmie presne rovnať počtu kopiek v kole — posledný tipujúci nemôže zadať tip, ktorý by toto spôsobil.</p>
 
           <p className="font-semibold mt-3">2. Hranie kariet</p>
           <p className="mt-1">Prvú kopku otvára hráč s <b>najvyšším tipom</b>. Každú ďalšiu otvára víťaz predchádzajúcej kopky.</p>
@@ -58,13 +58,13 @@ export default function RulesModal({ onClose }: Props) {
 
         <Section title="Bodovanie">
           <p>Po každom kole: ak sa tip <b>presne zhoduje</b> s počtom získaných kopiek → <b>10 + tip</b> bodov, inak <b>0</b>.</p>
-          <p className="mt-1 text-gray-400">Príklad: tipoval 3, získal 3 → 13 bodov. Tipoval 3, získal 2 → 0 bodov.</p>
+          <p className="mt-1 text-green-dim">Príklad: tipoval 3, získal 3 → 13 bodov. Tipoval 3, získal 2 → 0 bodov.</p>
           <p className="mt-2">Vyhráva hráč s najvyšším celkovým súčtom po 4 sériách.</p>
         </Section>
 
         <button
           onClick={onClose}
-          className="mt-4 w-full py-2.5 rounded-xl bg-slate-700 hover:bg-slate-600 font-semibold"
+          className="mt-4 w-full py-2.5 rounded-xl border border-gold/30 text-gold hover:bg-gold hover:text-table font-serif font-semibold transition-colors"
         >
           Zavrieť
         </button>
@@ -76,8 +76,8 @@ export default function RulesModal({ onClose }: Props) {
 function Section({ title, children }: { title: string; children: React.ReactNode }) {
   return (
     <div className="mb-4">
-      <h2 className="font-bold text-base text-green-400 mb-1">{title}</h2>
-      <div className="text-gray-200">{children}</div>
+      <h2 className="font-serif text-base text-gold mb-1">{title}</h2>
+      <div className="text-green-score">{children}</div>
     </div>
   );
 }