Add help modal and back button to score entry pages

HelpModal component shows game setup and scoring rules in a scrollable
overlay, triggered by a ? button top-right on Player Setup and Enter
Scores. Enter Scores also gets a ← back button top-left that returns
to the scoresheet without saving. PageFrame gains a headerActionLeft
slot to support the left-side button.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/App.tsx

@@ -59,5 +59,5 @@ export default function App() {
   if (page === 'title') return <TitleScreen onNewGame={handleNewGame} />
   if (page === 'setup') return <PlayerSetup onStart={handleStart} initialNames={previousPlayers} />
   if (page === 'scoresheet') return <ScoreSheet game={game!} onScoreRound={() => setPage('enterscores')} onNewGame={handleNewGame} onEditRound={handleEditRound} />
-  if (page === 'enterscores') return <EnterScores game={game!} onSubmit={handleSubmitScores} initialValues={editingRound !== null ? game!.rounds[editingRound] : undefined} />
+  if (page === 'enterscores') return <EnterScores game={game!} onSubmit={handleSubmitScores} onBack={() => { setEditingRound(null); setPage('scoresheet') }} initialValues={editingRound !== null ? game!.rounds[editingRound] : undefined} />
 }