Add heir management functionality in Falukant module

- Implemented setHeir method in FalukantService to designate a child as heir, including validation checks for user and child relationships.
- Updated FalukantController to expose the setHeir endpoint, allowing users to set heirs via the API.
- Enhanced FalukantRouter with a new route for setting heirs.
- Modified FamilyView component to include UI elements for setting heirs, with success and error feedback.
- Updated localization files in both German and English to include new translations related to heir management, improving user experience.

backend/migrations/add_is_heir_to_child_relation.sql

@@ -0,0 +1,9 @@
+-- Migration: Add is_heir column to child_relation table
+-- Date: 2025-12-08
+-- Description: Adds a boolean field to mark a child as the heir
+
+ALTER TABLE falukant_data.child_relation
+ADD COLUMN IF NOT EXISTS is_heir BOOLEAN DEFAULT false;
+
+COMMENT ON COLUMN falukant_data.child_relation.is_heir IS 'Marks whether this child is set as the heir';
+