Complete Tailwind CSS v4 migration using official upgrade guide (#96)

* Initial plan

* Complete Tailwind CSS v4 migration using official upgrade tool

Co-authored-by: axymthr <17650429+axymthr@users.noreply.github.com>

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: axymthr <17650429+axymthr@users.noreply.github.com>

Author: Copilot

web-backend/src/main/frontend/styles.css

@@ -1,3 +1,4 @@
-@tailwind base;
-@tailwind components;
-@tailwind utilities;
+@import 'tailwindcss';
+
+@source "../jte/**/*.jte";
+@source "../resources/templates/**/*.html";