Add multishop overwrite confirmation for category saves

[therampagerado]

Motivation

• Prevent accidental overwrites of per-shop category text fields when saving a category in the "All shops" multistore context by warning merchants and offering overwrite options.
• Provide choices to Overwrite all shops, Fill empty values only (preserve existing per-shop non-empty values), or Cancel before committing changes across shops.

Description

• Add constants MULTISHOP_OVERWRITE_ALL and MULTISHOP_OVERWRITE_EMPTY and multishop-aware save logic in AdminCategoriesController via an overridden processUpdate that blocks save when differences exist unless an overwrite mode is supplied.
• Add an AJAX handler ajaxProcessCategoryShopDifferences and helper methods (shouldCheckMultishopOverwrite, getMultishopTextFields, getMultishopTextFieldLabels, getSubmittedTextFieldValues, getCategoryLangValues, getTextFieldDifferences, restoreNonEmptyTextFields, isTextValueEmpty) to detect per-shop differences and to implement the "fill empty values only" behavior that restores non-empty per-shop values after a save.
• Expose template variables from renderForm and modify the category form template to add a hidden multishop_overwrite input, a confirmation modal, and client-side JS that requests diffs, shows the modal with affected shops/fields, and submits the chosen overwrite mode.
• Minor SQL safety improvement using bqSQL when building the category_lang select list for text fields.

Testing

• No automated tests were run for this change.

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Codex Task

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve clicked submit button before resubmitting

The submit handler prevents the native submit and later calls form.submit() after the AJAX check. A programmatic submit does not include the original submit button name/value, which AdminController relies on (e.g., submitAddcategoryAndStay vs submitAddcategoryAndBackToParent) to set the save action and redirect. In the “All shops” context this means Save/Save‑and‑Stay can be ignored or defaulted whenever multishop overwrite checking is enabled, because the resubmitted request lacks the submit button field. Capture the submitter (or inject a hidden input) before resubmitting so the controller can detect the intended action.

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