Improves MultiFlexi app guidelines and localizes SMS config

Expands documentation with schema validation steps and clarifies requirements for configuration files.

Updates SMS app JSON to schema version 3.1.0 and enhances localization for Czech language fields.

Promotes consistency and usability in app configuration and developer workflow.

Author: Vitexus

.github/copilot-instructions.md

@@ -1,5 +1,33 @@
 # Copilot Instructions
 
-All files in the multiflexi/*.app.json directory must conform to the schema available at: https://raw.githubusercontent.com/VitexSoftware/php-vitexsoftware-multiflexi-core/refs/heads/main/multiflexi.app.schema.json
-
 After every single edit to a PHP file, always run `php -l` on the edited file to lint it and ensure code sanity before proceeding further. This is mandatory for all PHP code changes.
+
+## MultiFlexi Specific Guidelines
+
+### Application Configuration (`*.app.json` files)
+- All files `*.app.json` must conform to the schema available at: https://raw.githubusercontent.com/VitexSoftware/php-vitexsoftware-multiflexi-core/refs/heads/main/multiflexi.app.schema.json
+- All files `*.credential-type.json` must conform to the schema available at: https://raw.githubusercontent.com/VitexSoftware/php-vitexsoftware-multiflexi-core/refs/heads/main/multiflexi.credential-type.schema.json
+- Always use schema version 3.0.0
+- Use `cmdparamsTemplate` with `${VARIABLE}` syntax for placeholders
+- Environment variables must follow the pattern `^[A-Z0-9_]+$`
+- Required fields: `$schema`, `name`, `description`, `executable`, `environment`
+- Use localized strings where appropriate
+
+### Validation
+Please ensure any changes to `*.app.json` files are validated. The MultiFlexi CLI validation command may not be available in all versions:
+
+```bash
+# Try MultiFlexi CLI validation (if available)
+multiflexi-cli application validate-json --file multiflexi/[filename].app.json
+
+# Alternative: Use online JSON schema validator or tools like ajv-cli
+# Install ajv-cli: npm install -g ajv-cli
+# Validate: ajv validate -s schema.json -d multiflexi/[filename].app.json
+
+# Alternative: Manual JSON syntax check
+json_verify < multiflexi/[filename].app.json
+```
+
+**Note**: The MultiFlexi CLI validation may have issues in some versions. Ensure the JSON is syntactically valid and follows the schema structure manually if validation tools are not working.
+
+