Add multishop overwrite confirmation when editing categories in all-shops context

[therampagerado]

Motivation

• Prevent accidental overwrites of per-shop category text fields when saving a category in the all-shops context by prompting the merchant with overwrite choices.
• Provide options to either overwrite values for all shops, only populate empty shop values, or cancel the save to avoid data loss across shops.

Description

• Add overwrite flow to AdminCategoriesControllerCore by introducing MULTISHOP_OVERWRITE_ACTION_* constants, a multishopOverwriteData holder, and an override of processUpdate() that detects per-shop differences via getMultishopOverwriteDifferences() and shows the form again when differences exist.
• Implement helper methods shouldCheckMultishopOverwrite(), getMultishopTextFields(), getAssociatedShopIds(), getMultishopCategoryLangValues(), restoreNonEmptyMultishopValues() and isMultishopValueEmpty() to identify changed text fields and to support the overwrite_empty behavior which preserves non-empty shop-specific values.
• Populate the template variable multishop_overwrite in the controller so the form layer can render UI, and add a hidden multishop_overwrite_action field plus a Bootstrap modal and JS in admin-dev/themes/default/template/helpers/form/form.tpl that intercepts submits and offers the merchant three choices: Save for all shops, Save only empty values, or Cancel.
• When overwrite_empty is chosen the controller saves the posted values and then restores previously non-empty per-shop values so only empty fields are filled from the all-shops submission.

Testing

• No automated tests were executed for this change (no test run was requested).

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[chatgpt-codex-connector]

Preserve clicked submit action when confirming overwrite

When the modal confirmation triggers, the handler calls $form.submit() programmatically, which does not include the originally clicked submit button name/value. As a result, if the user clicked "Save and stay" or "Save and back to parent" the POST no longer contains submitAddCategoryAndStay (or similar), so the controller treats it as a normal save and redirects differently. This regression only happens when the overwrite modal is shown; consider capturing the clicked submit button (or adding a hidden field) and re‑submitting that action along with multishop_overwrite_action.

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