Add confirmation before erasing game history on import or position edit

[Copilot]

• Add isEmpty() method to MoveTreeWidget to expose whether the move tree has moves
• Show confirmation dialog before "Import moves" when the move tree is not empty (contextual menu via importMovesCallback in analysis-board.js)
• Show confirmation dialog before "Import moves" in the dropdown menu (move-history-drop-down-menu.js)
• Show confirmation dialog before "Edit start position" in the dropdown menu (move-history-drop-down-menu.js)
• Show confirmation dialog before "Edit start position" button in settings panel (settings-manager.js)
• Extract UI.openWithConfirmation(shouldConfirm, confirmText, yesButtonText, openFn) helper to eliminate duplicated confirmation pattern across all call sites

[benckx]

@copilot how could we avoid this copy pasta?

    const openImportMovesModal = () => {
                UI.showModalByName(Modals.IMPORT_MOVES, () => {
                    new ImportMovesHandler(this.#boardGui, this.#moveTreeWidget);
                });
            };
            if (!this.#moveTreeWidget.isEmpty()) {
                const text = buildSimpleSpan('Importing moves will erase the current game history. Continue?');
                UI.showConfirmationModal(text, openImportMovesModal, 'import', () => UI.hideModal(null), 'cancel');
            } else {
                openImportMovesModal();
            }

[Copilot]

@copilot how could we avoid this copy pasta?

...

Extracted the pattern into a new UI.openWithConfirmation(shouldConfirm, confirmText, yesButtonText, openFn) helper (commit b14c63e). It handles the buildSimpleSpan + showConfirmationModal vs direct call logic internally, so each call site now reads as a single expression:

UI.openWithConfirmation(
    !moveTreeWidget.isEmpty(),
    'Importing moves will erase the current game history. Continue?',
    'import',
    () => UI.showModalByName(Modals.IMPORT_MOVES, () => new ImportMovesHandler(boardGui, moveTreeWidget))
);