Add Thymeleaf landing page and error page

[Copilot]

This project had no web UI — only bare REST endpoints. Adds a styled landing page and a custom error page using Thymeleaf.

Changes

• pom.xml – Added spring-boot-starter-thymeleaf dependency
• WebController – @Controller mapping GET / → index Thymeleaf view
• templates/index.html – Landing page listing available endpoints with links to Swagger UI and the version endpoint
• templates/error.html – Error page rendering HTTP status, error name, and message via Thymeleaf model attributes (${status}, ${error}, ${message}); includes a "Back to Home" link
• WebControllerTest – @WebMvcTest verifying GET / returns HTTP 200 and resolves the index view

Spring Boot's BasicErrorController automatically populates the error model and routes to templates/error.html for all 4xx/5xx responses.

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