Implement manual and automated tests for auto-save countdown feature

- Added a comprehensive manual testing guide for the countdown functionality in `test_countdown_manual.md`.
- Developed a detailed Selenium test suite in `test_countdown_selenium.py` to verify countdown behavior, including server accessibility, login, countdown progression, and UI elements.
- Created a minimal Selenium test version in `test_countdown_selenium_minimal.py` for quick checks without complex setup.
- Introduced a verification script in `test_countdown_verification.py` to analyze code correctness, including server checks, build file existence, and countdown logic.

migrations/remove_difficulty_level.sql

@@ -0,0 +1,5 @@
+-- Migration: Fjern difficulty_level kolonne fra material_packages
+-- Dato: 2025-11-14
+-- Beskrivelse: Fjerner sværhedsgrad fra SmartPakker da det ikke bruges længere
+
+ALTER TABLE material_packages DROP COLUMN IF EXISTS difficulty_level;