feat(Tournament): add doubles tournament support and related UI enhancements

- Updated tournamentController.js and tournamentService.js to include isDoublesTournament parameter for tournament creation and updates.
- Modified Tournament model to add isDoublesTournament field, allowing differentiation between singles and doubles tournaments.
- Enhanced frontend components (TournamentConfigTab, TournamentParticipantsTab, TournamentTab) to support doubles tournament configuration and display.
- Added internationalization keys for doubles tournament labels and hints in multiple languages.
- Improved participant assignment logic for doubles tournaments to ensure proper class assignments.

backend/models/Tournament.js

@@ -51,6 +51,13 @@ const Tournament = sequelize.define('Tournament', {
         defaultValue: null,
         comment: 'Anzahl der Tische, auf denen gespielt wird'
     },
+    isDoublesTournament: {
+        type: DataTypes.BOOLEAN,
+        allowNull: false,
+        defaultValue: false,
+        field: 'is_doubles_tournament',
+        comment: 'Turnierweite Markierung fuer Doppel-Turniere'
+    },
 }, {
   underscored: true,
   tableName: 'tournament',