Updated learning.py

Refactor to dispatch dictionary, remove unused imports

- Replaced 500+ line elif chain with ALL_TOPICS = {**BASIC_TOPICS, **INTERMEDIATE_TOPICS}

- Single dispatch: teach_func = ALL_TOPICS.get(selected_topic)

- Removed 7 unused imports (run_practice_session + 6 validator functions)

- Version bump: 2.1 Hotfix → 2.2 General Availability"

Author: acubura