Migrate server-side views from JSP to Thymeleaf

[goekay]

We currently render web UI views with JSP/JSTL. This makes consistent output encoding difficult and increases security risk when raw EL output is used in different contexts (HTML, attributes, URLs).

Motivation

• Move toward a modern Spring-aligned view stack.
• Simplify template composition and long-term maintainability.
• Improve security safety baseline.
• Reduce manual context-encoding mistakes in JSP EL output.

Out of scope

• UI redesign.
• Controller endpoint redesign.
• Business logic changes unrelated to templating.

Reference: #1733 (comment)

[juherr]

Awesome!