feat(helpdesk): add handover protocol schema editor to software products

Add a JSON-based schema editor as a new tab on the software product edit
page. Admins can define protocol fields (heading, paragraph, text,
textarea, number, date, checkbox, select) via a structured UI with
add/remove/reorder controls. The schema is stored as a versioned JSON
column on the product row.

Includes DB migration, server-side validation (type whitelist, key
uniqueness, key format, max 50 fields, select option checks), i18n
(DE+EN), CSS, and 10 PHPUnit tests.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

modules/helpdesk/migrations/004_add_handover_protocol_schema.sql

@@ -0,0 +1,2 @@
+ALTER TABLE `helpdesk_software_products`
+    ADD COLUMN IF NOT EXISTS `handover_protocol_schema` TEXT DEFAULT NULL AFTER `name`;