Export game history as CSV or JSON

[BySplashGm]

Description

Allow players to export their stats and game history for external analysis or backup.

Acceptance criteria

• Export button in StatsScreen (share icon in app bar)
• Generates a JSON file: all games + throws for the selected player
• Optional: CSV export for spreadsheet use (one row per throw)
• Uses share_plus to surface the OS share sheet
• Works on both iOS and Android

Fields to export

game_id, date, variant, players, winner,
throw: round, dart_number, raw_value, multiplier_type, score_value, is_bust